Contrary to popular belief Bubble Butts are ~not~ big asses. Bubble Butts are round like a globe, usually complement a slender/slim body, they are very tight and firm unlike the one's on fat asses...

A nicely rounded behind.

A big, fine perfect round ass. It is not huge and gross, just bigger and better than average size. Every girl should have one of these. These anatomical masterpieces can give many guys an erection.

A perfect butt; smooth, round and soft. Often used in a homosexual context.

an ass is very round and "bubbly"

A word used to describe a nice, large ass

Her ass is very round and "bubbly".