IF the statuettes in question (such as that depicted at
http://www.jlc.net/~brian/art/prehistoric.html) were part of a religion, it
would not be required for women of that physique to be common in order for
statues to become widespread.  It could quite possibly have been only one
woman (literally one) with legends sprouted up about her and then once the
statues were around more could continue being made for quite some time
after her death.

Of course the faceless nature of the statue does imply lack of individual
identity so this is unlikely.

However even more plausible is the possibility that some tribes considered
having an enormously fat woman a great status and/or fertility symbol (or
it could have started as a status thing and evolved into fertility
spirituality).  There are peoples in the world NOW.  Not long ago I read
about a people who consider enormous fatness a sign of great status for
men, and so their families will pick a favored son and force-feed him huge
amounts of milk and make sure he is kept very sedentary; there are actual
contests to see who has the biggest (and therefore most desirable) son.
Most of the people in this culture (whose name is escaping me) are not
particularly fat.

The theory that the statues are a form of pornography is not particularly
far-fetched and I'm amazed that anyone scoffs at it.  Males appear to be
genetically prone to being sexually aroused by visual stimulus, and
masturbation (especially among young men) is about as natural and universal
as anything in human behavior.  "Porn is about profit" is an interesting
political formulation but it really doesn't apply to reality.  Every young
man with a modicum of artistic talent I ever knew loved to draw pictures of
naked women.  There's a certain age where the fascination with this kind of
thing is absolutely ubiquitous.  I don't know that I would endorse this
theory but it's really not silly or outrageous.