I measured them once when new. OEM rear pads have 9mm friction material on the rear. OEM front pads have 6mm.
I notice that some aftermarket companies provide what look like 6mm (front pads) for the rear calipers, and may not even produce pads with 9mm thickness. The rear pads tend to wear out much faster (at least in the UK when the bike is ridden in winter months, due to the amount of crud that gets kicked up onto them. So it is handy having the extra thickness.)
On at least one of my 1100s, the backing plate had the same dimensions for front and back calipers. Don't confuse the two !
Differences - rear pads have a notch in the tab end of the backing plate, front do not. You could get away with putting a notched pad in the front, but you wouldn't get away putting a pad without the notch in the rear - the brakes won't release properly.
Some aftermarket manufacturers include a removable heat resistant layer for all of their pads. These aren't needed for the front brakes, and although it is possible to fit them, the extra thickness will cause the front pads to drag which will activate the rear brakes if you have the CBS on your ST1100.
