Here's a very brief overview of how to re-size thighs (or other armor, in my case it was thighs!) with a mix of scraps (always good to save what you can from trimming/assembly!) and some ABS paste. You will also need:
* some spare ABS sheet (you can but it online, some use "For Sale" signs)
* glue (E6000 or other)
* sandpaper in several grits (I used: 80, 150, 320, 600, 800, 1000, 1500, 2000, and 2500)
* a dust mask (ABS dust is probably bad for you)
* ABS polish (Novus