Also I never know which button to press. Sometimes the big button is the one to press because UX people say it's the most visible, so that produces the smaller

Also I never know which button to press. Sometimes the big button is the one to press because UX people say it's the most visible, so that produces the smaller flush. If you want a bigger flush, use the smaller button. Sometimes (more logically) big button = big flush. With a single control, it's more commonly hold for short flush, release for long flush, which is just madness.