You can probably tell which one is for the smaller bust...
It's not a problem for people with a decent sized breasts, but being rather flat chested, I find it very hard to find a bikini that doesn't make me look completely prepubescent. If you're in the same boat as me, here's a few things I've learnt:
Avoid bandeau bikinis - I don't know what happens, but you become an ironboard as soon as you slip on one of these. It might be the way it wraps around your boobs.
Opt for triangle bikini tops - the triangular shape gives you some definition.
Wider straps - create an illusion of a plumper bust. Thin stringy straps seem to highlight how flat chested you are.
Cups - only work if they are in your bra size 28B, 32A, 34AA etc. Don't buy a size 8 padded bikini top if you don't fill it properly.
Decoration - frills and ruches are supposed to give the appearance of curves. I'm not really sure if this is true, but there's no harm in it.
Light/bright colours - take away from your shape. Weirdly, neon colours make eyes forget that you don't have a pair of bouncing watermelons. All people think about is the colour.
Plunging V-necks and halters - are also meant to create an illusion of curves?
Well, that was a short article. It's a shame there are so many bikinis for the 'bigger woman' but so little choice for people on the opposite end of the spectrum. I am still on the quest for the *perfect* bikini. I don't know if it will ever be found.
PS Onepiece swimsuits are just as hard to find! Most of the rules above can also be applied.