Had some free time and did some measuring with my calipers
Without a backstrap on both pistols measured the same at .999"
I measured the width of each backstrap at its widest point. S= 1.133", M= 1.270", L=1.357"
I then measured each pistol with each grip (S,M,L) and measured at the same location for each.
.40 cal S= 2.153", M= 2.111", L= 2.286"
.45 cal S= 2.236", M= 2.283", L= 2.400"
What's interesting to note is the 40 cal with the small vs the medium grip. The small gives you more length front to back with a narrow width. Medium is shorter front to back, but more width.
YMMV