I'm glad I'm not the only one with this confusion, my friends mock me relentlessly based on past discussions we've had.

Since on some designs the smaller button cannot be pressed separately and therefore corresponds to a big flush, since both parts of the button are depressed at once; it stands to reason small will always be bigger flush, which I rationalise as urination being the more common operation and therefore deserving the larger button in order for the user to press more easily. However it's actually the case that big is usually the larger flush, which suggests there's some magic button size ratio at which their meaning crosses over.

I think this goes to show how difficult UX is.