First, read this (the whole thread, but mainly Steven's pictorial example) so you know what kind of ill-posed problem you're up against.
I would think using the distance transform ( bwdist ) would be a good start. You could get the distance xform, threshold it and then erode the whole thing by a disk strel with a diameter (or radius?) the value of the threshold.
You might also be able to do a delaunay triangulation and then only keep small area triangles.