What do you think are the sound effects for expansion/growth?

[Lightfoot]

Although sometimes obvious sound effects can be used for the growth if you are trying to be funny. Or if the growth is in really small amounts, or even out of view. "grow" or "swell" might make sense in those cases.

I think one sound effect wouldn't work that well. It depends on the speed of the growth, and the amount of the growth. I've always liked "Ka-Boob!" if the growth is fast, and the person has grown a lot.