Ahaha, that is one of the things that is bugging me about the Silurians, because they are reptiles so why do the females have breasts. There are a couple of fanon theories floating around: one, that that they actually do nurse their young (which does not jive with my headcanon); two, that they just have really well developed pectoral muscles which in the right clothing, can look like breasts (if so then why don't the males look busty too?!)

Of course the real reason is because none of this ever occured to the design team.