You don't get enough time to regret your words because as soon as that magic energy hits you, you feel your body warm up all over. Your face pushes out to create a dragon muzzle and you grow massive horns on top of your head. Your skin is replaced by white scales, they're strong as steel. Your hands gain deadly claws and so does your feet. Next you grow enormous wings and then a tail with a spaded tip.
As uncomfortable as those changes were, they're nothing compared to what happens next. Your bones and organs reshape all over to get you on all-fours like how a proper dragon should be. Organs become bigger and move places, bones grow and change until you're a mighty beast. You were so focused in your transformation that you failed to realize you're now on a big open grass field in the middle of a forest, the witch is gone. You take some time to check out your new body. You're a massive dragon. Your scales are white and your belly, which is similar to a snake's belly with scoots is purple. You have a mohawk mane on top of your head and you can see you got a long dragon mustache. Considering the heavy, pendulous tool you're feeling between your legs, you remained male.
Even though you're not used to this body, you can tell you're incredibly muscular all over. The witch is not done with you though become soon enough you feel another wave of change coming. This one is easier than the last ones, because this time you simply grow bigger. You watch as the ground gets further away from you until you're a behemoth of a dragon. It's not over yet, you feel more growth coming... but this time it's very different.
Terrified and confused due to your changes, you attempt to run. Your run is quite undignified due to your belly, and each one of your steps is making the ground shake. You left gigantic pawprints on the earth and a trail of flattened grass and trees due to your gut. Wait, you're a dragon, right? You can just fly out of here! Your wings are just as wide and as big at the rest of you, you begin to flap them but obviously, you're not going up. Not when you're this fat. You begin to flap them as hard as you possibly can, until the point of exhaustion. You had to give yourself some credit here, because you actually got off the ground for at least half a minute before crashing down on top of your belly and causing an earthquake.
The constant moving of your belly caused it to slosh all over inside, you can't help but belch an extreme amount of fire into the trees in front of you. Causing a furious, catastrophic forest fire. That certainly did not go well. You growl in frustration, wondering where that witch its. Your question is answered when she shows up out of thin air on top of your draconic muzzle, a smug look on her face.
''Did you know dragons could be THIS fat, fool?'' she casts another spell and you're both teleported to a cave.
The cave feels endless. It's not as tall as you are so you can't fully extend your neck when standing up, but the widtht certainly makes up for that. You can stay in it laid down just fine. What truly surprises you though is the huge ocean of gold that feels almost as endless as the cave. Actual gold, coins, chests, gems, ancient artifacts and enchanted weapons as far as your dragon eyes can see, and you're laying on top of the biggest mountain of gold in the cave.
''I... you... why? How?!'' You find out you can still speak, your voice is horrifyingly deep and intimidating even when you're confused.
''Silence, lizard! Due to your insolence, from now on you shall guard this horde as a beast for five thousand years.'' A gigantic, gold collar with diamonds on it is attached to your neck. Other jewelry is also put on your other limbs such as your legs and tail, giving you an even mightier look.
''A single coin is stolen from this horde, and I shall add a thousand years to your punishment! Maybe in those years, you will learn some respect.'' Without another word the overweight witch is gone.
That was certainly something. You walk and look around for a while, your steps toss things worth fortunes all over the place. You definitely can't fly in here. There is nothing for you to do. You eventually find out and exit which leads to other areas of the cave where there is no gold. You doubt you can even fit through that exit and you know the witch most likely has taken other measures to keep you where you are, so you don't even try. If you had to guess, you must be incredibly deep underground. Perhaps you could dig up through stone? No, the witch most likely has made other ways of escaping impossible, not to mention your weight. You're sure as hell not going to try and breath fire on the treasures to damage it and end up with a bigger punishment.
While walking through that endless ocean of gold, you find out that you have knowledge you didn't have as a human. Such as flying, how to breath fire, and how to battle as a dragon. You supposed it came with the body. Though the moves and attacks in your head are not going to look identical to what you're thinking due to your new weight. Without nothing else to do, you go back to the biggest mountain in the horde and lay on top of it, soon falling asleep.
You find out your sleep is very deep and full of dreams that you can control. You wake up to shaky steps on the gold coins, you're surprised by a tall man in full gold, plate-armour. Well armed with a magical sword and shield. If you were still human, you'd be impressed and scared of this man. But now, he's incredibly tiny and insignifcant. You're not sure how long it has been, but it would come as no surprise at you if you just slept a decade or mode, and judging by what the man is wearing, you've been teletransported to a medieval era.
''...B-Beast! Dragon! I-I... I-I've come to slay you!'' He's shaking, completely terrified of you. For good reason.
He wastes no more time, with a warcry, he bravely charges at you to strike your face. It's comical, when his enchanted sword hits you, it's like it hit concrete. It bounced off and nearly made him fall on his back. He stops dead on his tracks, now even more scared, yet still he doesn't run. The whole situation is a minor inconvenience to you, you'd rather be asleep. You simply bring a claw near the man and flick him off miles away to hit a stone wall. He's dead instantly. His items and equipment making the horde a tiny bit bigger. A legend gone just like that. You go back to sleep without giving that event much thought at all.
That's your life from now on, you sleep and fight here and there. The fights simply bother you and to make matters worse, you sometimes wake up quite grumpy because of the adventurers that come seeking your treasure. Legendary knights, wizards, adventuring parties, armies of all kind have been crushed by you like nothing. You rarely ever have to stand up or breath fire. Though you admit you actually enjoy when you have to put effort into a battle, you get more creative with the ways you defeat the pesky adventurers as time goes on.
Sometimes you even speak with them to put on a show to amuse you. You have swallowed adventurers whole, threw them away with your tail. turned them into ashes, you have even sat on an entire army to defeat it. You suppose your weight has its uses. You can't really be blamed, they're the ones that come to attack you. You spare certain warriors when you feel like it, some manage to run away to tell the tale. The legend of the behemoth, fat dragon that sleeps on an endless horde of gold deep underground only grows.
You're like a secret level 999 final boss from games you used to play as a human and no one has even been able to defeat you. Dents in your scales is all you have to show from all the legends you defeated. The horde is significantly bigger because of you, and you're still not sure how long it has been. You don't have to worry about being cold with all your fat, and if you're hungry, it doesn't take long for you for a snack to show up. You wouldn't be surprised if there are some people still alive inside your belly, it doesn't matter to you. Have all these people interacted with the witch at some point too? Do they respawn? You don't know, and you don't care. You go back to sleep.