It's missing some of the argument. Part of the idea is pre writing humans passing down successful pregnancy and reproductive information. Women obviously died from childbirth back then as they do now. One theory is that these women were trying to pass down some information of perhaps considered successful childbirth.

There have been times in relatively recent history where incorrect information about pregnancy was being passed down, plus a larger woman would be getting more nutrients in that time than malnourished women. An easy conclusion to make may uave just been to eat alot and be larger.

Speculation and a hypothesis are two very different levels of certainty in a claim. I suppose, though, that this area of research is somewhat forced to use more certain language than other areas would be comfortable with, given the same quality of evidence. Recognize that "we're just guessing here" also applies to the claim in the meme.