Mary rubbed her half-hooves over her round and very pert udder, playfully flicking her teats. Her udder came out of her as a half-sphere. She was thinking that she'd like to feel it swell up again.
"Rob, would you do me a favor? Blow into my teats." Mary placed her pink rubber udder in front of Rob's fat face. Without a second's thought, Rob stretched and snagged a teat. Immediately he start to suck. Mary watched in dismay as her udder started to deflate. She whacked Rob on the head.
"Rob, don't suck, blow!" Rob quickly stopped his sucking and started to blow. Mary mooed as she felt warm air suddenly coming into her udder. It slowly regained it's pertness and more. The became more taut and began to balloon out. Mary rubbed and caressed her swelling udder, feeling a different kind of pressure than the milk before.
Rob kept blowing into Mary, filling the rest of her more slowly than the udder, but still swelling the rest of her up. She became plump, then husky, then fat. Mary pulled herself away from Rob, a fat rubber cowgirl now.