Jeff's first thought was of his lifelong friend Danny.
Danny was a shy, awkward boy who wanted to have fun but had trouble making friends. What didn't help was Danny's genetic situation. Even though Danny had grown up in a family of tall, rock-climbing jock brothers and tall, gorgeous sisters, he himself was cursed with completely different genes. As he went through puberty, Danny realized he was going to have a very different fate from his gorgeous, blessed siblings. By 15, he was only 5'5", and he begged his parents to tell him what went wrong and how he could fix it before his puberty finished and his bones were locked in place forever. They sadly had to explain to him that his height was rooted totally in the wonderful lottery of genetic combination, and there was nothing he could ever do about the body he was born in. The best he could do was accept the beautiful decisions his body made about how to grow and not compare himself to his more lucky siblings.
Danny obsessed in misery about his height, his horrible fate which wasn't his fault at all. What was worse, he had a constant craving for the kinds of ridiculous junk food his tall, active brothers and sisters loved to eat. Since Danny had a much smaller skeleton and was too embarrassed to work out and have fun as his luckier brothers and sisters could do without a second thought, the evidence of Danny's cruel fate began to appear as a new humiliation -- first around his tummy, then around his butt and thighs, then around his neck. Danny wouldn't just be pathetically short, he was developing habits that would make him a chubby, lazy boy. His poorly built skeleton would become wrapped in the "outfit" of a chubby 15-year-old who was beginning to give up anyway. By the time Danny was 16, he was desperate with misery and humiliation about the body his parents' faulty genetic combination had built for him, but nothing could've prepared him for the journey of being a fat boy! As cruel puberty continued, Danny had developed a huge, soft tummy he couldn't dream of hiding. His butt was similarly huge and imposing. Worst of all, he had quickly developed breasts, which hung off him and constantly called attention to his pathetic body no matter how badly he wanted to hide them.
Jeff, of course, noticed all of this. He always liked his friend, but as he compared his own successful puberty to Danny's growing humiliation, he always felt a strange fascination. Jeff had always been a lanky, active kid, and couldn't imagine what it was like to be short and fat with no way to fix your stubborn body.
He knew what his first transformation would be. He had to turn into Danny and feel every moment of life as a short, chubby dude, no matter what isolation and humiliation it would bring.