The bullet hole decals are on a single texture sheet in:
FPS Creator\Files\gamecore\bulletholes
These aren't the same as decals like splinters etc, which are animated and bulletholes.dds is more of a sprite Atlas. There are no decalspecs for these so I don't know that there is any way to increase their size in game (you could try putting a decalspec.txt with scale field in there and see what happens). It is probably your best bet to look to the materials index for this than weapons themselves.
There
may be some way to override decal impact in Airmod, though not even sure about that since it is probably more dependent on material index in the segment or entity. I believe there may have been some discussion about melee weapons and how you would want a slash decal instead of a bullet hole for an axe for instance so maybe tracking down these threads might help.
Pretty sure flak would leave a larger scorch mark than a bullet hole so thats something else to look at. All just guess work on my part as all this is something I never looked too closely at myself
The only person I ever saw changing these for sure was Errant but he didn't exactly change scale in any way.
http://forum.thegamecreators.com/?m=forum_view&t=102411&b=24
