Certainly, there are women who have, by virtue of their size, difficulty in finding clothing that doesn't fit them rather tightly. I think, though, that it is easy to distinguish such woman from those who don't have size issues but nonetheless wear very snug and revealing attire. I have no problem making allowances for the bigger lady who is obviously trying to dress modestly but finds her efforts confounded by her size. Again, a woman in such a circumstance is, in my experience, easy to identify and not particularly provocative.
My Mom is a bigger lady and I have never seen her wear anything that could be construed as immodest. And she doesn't seem to have any problem finding very nice but very modest clothing. I'm not sure, then, that your assertion that bigger ladies will struggle with modest dress holds much water...
Selah.