There’s no reasoning behind it, just rationalization. Nobody starts out thinking reasonably, and concludes that abortion is okay for a baby that is two pounds, or two months old, but not one that is eight pounds, or eight months old. The only way that you get to that position is by wanting to find some way to rationalize abortion in face of the fact that looking at an aborted eight month old, eight pound baby is disgusting and revolting, and undeniably wrong. So, their brain comes up with a semblance of reason that is just enough for them to suspend their rational thinking in favor of.
It’s pretty much the same way that fiction writers have to make unbelievable things just believable enough so that the skeptical audience suspends its disbelief. If the audience didn’t want to believe the fantastic thing, their reason would make them scoff, but since they are willing participants in the fiction, wanting to enjoy it, then they accept the semblance of believability, just to quiet their rational objections. By the same process, our brains can wallpaper over our objections to any evil thing that we want to commit. I’d say it’s man’s specialty.
I think they are just using the large size to prove minimum gestational age. If you read the article the expert also cited muscle mass, hairline, and scrotum as proof. I don’t think its ‘sizeism’.