How do I know if my bikini bottom is too small?

Anonymous
I got a new bikini and my top was large, but when I looked at the large bottoms they seemed huge, even though I have a sizeable butt. I got the mediums and didn't try them on because I was pressed for time and they can't be returned. I can't tell if they're too small because they fit comfortably, but my butt is perfectly outlined and if I'm moving around a lot the back and shift down enough to show a little bit of my butt crack :p I don't know if this means it's too small and I'll shock people if I wear it , but I have board shorts that I could wear and it'd still be a cute outfit, just not a bikini.

P.S, not posting a pic of my ass -.-
How do I know if my bikini bottom is too small?
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