I agree that the nymphs' curves need not mean that they are pregnant, it may just have been "normal" for the maker. And as you show, one needn't look far to find parallels. Any type of imagery that features nude people will do.
As in the example above, nude people in Medieval art are often "souls" or embodiments, both of which wear no clothes to illustrate their disconnection from the mortal world. But the "why" of the nymphs' nudity is a separate question.
Now to get back to body shape, there's more going on though. Many of the nymphs, with their large heads, stocky build and bumpy legs, have more in common with children than adults. Especially if you filter out those breasts that were added later.
Then on the other hand, they may be a derivation of a certain style of drawing the naked body which was already present in Carolingian works, but also in many later examples. This one from early 14thC Flanders. Note how not much change is required before you are unable to tell male from female.
And then there's the absurd "rules" the VM nymphs follow. It's a complex matter and I'm far from understanding it well.