1) A girl who is so "built" (Boobs, Hips, Butt) that it makes her (regular size) bikini look small or shrunken.
2) A girl who buys a bikini one size too small.
A girl can technically shrink any type of clothing.
(At the beach)
1) Damm, you see that girl shrinkin that bikini??
2) Past tense, Did you see those shrunken bikinis at the pool?
3) Person 1: girl, you Shrinking' that Bikini