Not to belabor the point, but breaking off pieces around the aperture of the
shell would actually reduce, not enlarge, both the size of the aperture, and
the size of the overall shell, including the available living space within.
It is true that hermit crabs do sometimes pick up shells that are quite
badly worn and/or broken, but they seem to prefer shells in better condition
when available, and in most localities, most of the shells inhabited by
hermit crabs are in good condition - often good enough to be of interest to
collectors.  In any case, reducing the size of the shell they have moved
into certainly would not be in their best interest.  There are two major
groups of hermit crabs (I don't remember if these are suborders, or
superfamilies, or what).  One group has wide, flat claws which are quite
powerful, and theoretically could break the lip of a shell (though I doubt
they could break the columella).  The other group has slender, forceps-like
claws, suitable for transferring bits of food to the mouth, but not suitable
for doing any major renovations to a newly acquired home.  Finally, I have
kept terrestrial hermit crabs, and none of them made any efforts to alter
the shells in which they lived, even though some of those shells were rather
thin land snail shells which could easily be broken.

Paul M.