The Bat's Magically-Embiggened Breasts Traveling the Multiverse chapter 16 by dellobenedetto

Well, you did promise to Rouge that you'd make her boobs bigger... so best you get that out of the way, huh? Fortunately, you have a number of size-altering spells in your repertoire, including a few that only enlarge part of a target, and not the whole thing. Preparing onesuch, you place your hands on the soft mammary wall, and extend your magic out to make sure you have both breasts caught in it. With that assured, you began muttering the incantation. It took a few moments... but gradually, you felt the world begin to shift as the breast grew, enlarging the milk-lake cave you were within. You had to shift your position on the spongy floor occasionally to stay next to the wall—not so much to keep the spell going, but moreso to ensure you were ready to make it stop.

Meanwhile, on the outside, Rouge felt a tingling sensation, and cupped her breasts to feel them enlarging beneath her hands. "Hmm... not bad! Not sure what you did in there, cutie, but it's working great! Might need to replace my suit's breastplate if I want to fit these babies in it~"

You took that as a sign to pause, reaching back into the spell to deactivate it—you're pretty sure Rouge's boobs are, what, three or four cup sizes bigger than before? You made sure to slip another spell into the lattice, a weight-reduction spell, to ensure their new size wouldn't impede the bat's flight—nor damage her spine—out of common courtesy. Now for the milk... That would be simple enough. You could see the milk ducts high above the lake, dripping feebly at the moment, but it wouldn't take much effort to "turn on the faucet," so to speak. Though maybe it wouldn't hurt to take a break and figure out something for yourself in here... especially with how spacious her boobs were now.