Bubble butts are proud and stick out like a bubble, announcing to the world, "GRAB ME, GRAB ME!" They are more a variation on "cherry butt" which has an even distribution above and below, side to side. All "perfect butts" are some variation of the cherry or bubble butt. Wife's got one.
Scientific observation has concluded there are four distinct gluteal formations: square, round, V-shape, and A-shape.
However, in-field testing has proven there are significantly more.
bubble butt or cherry - even distribution all around.
peach - Also a variation of cherry or bubble butt, defined by the perfect symmetry and roundness from back to thigh. A Peach butt graduates from a cherry with a distinct distribution of fine white fuzz.
ghetto booty - Big all over. Just . . . .too big to identify, probably a variation of pear/OBD below.
heart - Heart shaped, tapering in to the thighs at the bottom. A little topheavy, similar to the apple, but gives room for gravity to work it's magic over time without losing it's attractiveness.
apple - Cut an apple in half vertically. Turn it upside down, with the stem hanging down, and observe the profile. Gentle outward slope from the waist terminating in a roundness at the thighs, the apple is a standard perfect butt. Argued to be the same as the heart butt. Not an argument I want to pursue, because just thinking about it, I can't keep my mind on topic.
PEAR OMG, stay away from this one . . . a pear butt may develop long before gravity has had a chance to do it's evil work, but most ghetto booties will eventually develop into pears with sufficient time. It's ever-widening girth will drop and expand with each passing day and eventually develop into the shape of a pear.
Here we call it "OBD" (Oregon Butt Disease.) It's truly a tragedy, someone should open a non-profit fund for the carriers.