I do not know much but I do not think that pairs are around the same size. I believe it has more to do with who's gravid and if a male steps up(he may or may not be fertile...at the time). But they normally keep trying. Second thought you had a fertile male at 3-3.5 inches? If so were they siblings or just didn't grow? And if you have another fertile male at 3.5 and he does not grow anymore then your doing something wrong or buying fish from the wrong person.