Males are sometimes smaller but don't count on the size alone to figure out. Male frogs usually have black markings on their arms. They are also the vocal ones. If you should ever find two frogs clopped together the top ones the male. The black markings referred to are called nuptial pads and are used for the male to grasp the female but this does not go to ALL frog species. The majority of frogs only develop them during the breeding season anyway. Most males are smaller than females but not always. My Gargantua was a big boy and he was larger than all of the females I have seen. Any frog that calls for a mate is a male. Distress calls don't count, females can make those as well. Males are often more brightly coloured but this is not universal. And yes, if you find two frogs mating, the male is always on top. Another good indication is the size of the ear drum. Males will have ear drums larger than their eyes, while females will be the same size or smaller.