In the following I will try to explain the reasoning that could of been used behind such a choice in two main sections. Design, the reason it would of been important from a design standpoint, and scientific/biological the reason why it might actually make sense logically.
What would be a better feminine characteristics to use? Like a previous answer pointed out, you could just put some lipstick on a male model and call it a female of that species, but you would be laughed at. You presumably want the sexes to be easily differentiated, maybe you want them to have some sex appeal, how do you do this better than adding breasts? They are already designed as humanoid, breasts are a defining characteristic of humanoid species, by definition, so why would they not have them (unless the designer specifically wants the species to be extra alien and unrelateable)?
Also, this is a problem struggled with by all of scifi, even novelists to a degree. If you make something truly alien in looks, actions, or thought, humans will inherently fear and hate it. People inherently fear reptiles and insects, and they are not nearly as alien as aliens might be. This is typically not the desired response for most scifi characters. An author/designer needs to do things like add relatable emotions, human eyes, and breasts to characters so that you (as a human) can relate to them. This is exacerbated by non-mammalian species, you need to add in even more obviously human features to a reptile-person to not cause an intrinsic hatred/fear response.
These are all intelligent species, who live in large groups, presumably with large brains that take loads of food to grow and maintain, just like humans. It is not completely ridiculous to assume that not only breasts, but large ones are almost a prerequisite to becoming an intelligent civilization. Because the children need more care, some form of mother's milk is necessary, and since this is an important aspect of raising a child it becomes important that they are large to signal mates of her ability to feed their child. There is also the aspect that as they evolved intelligence they probably formed into tribes, where a single mother with the ability to at least temporarily take care of other children in the tribe would be a huge benefit.
We can also approach the question from another angle. Humans are incredibly unique because the male of the species actually has a say over who he mates with. Typically, they fight for resources (aka prime land for birthing/living/grazing on) and then mate with any females who choose this land OR they compete directly with other males to showoff their prowess or beauty to attract females to them. Many believe that the start of civilization is when this behavior stopped (possibly even by definition, aka Civ. is simply impossible without this). When men stopped being completely disposable, and most of them them instead of a tiny tiny minority could reproduce, then many times more work could be expected from these males making the construction of civilization and the acquisition of enough food to make future survival reliable. This has a side effect that men get the social ability to turn down procreation with someone, meaning women had to work at attracting mates. Hence why they are one of the very few species who have exaggerated feminine characteristics.
Basically, we just do not know enough about potential intelligent aliens to have any idea what characteristics they might share with us. Humans have some unique features, but there are some very good reasons to believe that these would be shared with other similar alien species.