In your home game, would you allow a medium character to wield a small size double hackbut the same way a large character can wield a medium sized double hackbut?
This double-length rifle uses a pair of trunnions to mount its barrel into a swiveling mechanism fastened to a lightweight, two-wheeled carriage.
It takes a full-round action to set up the carriage. The carriage has a hind leg, allowing the wielder to wheel the device about and immediately prop it for stability during combat. Unlike other two-handed firearms, you must fire the double hackbut while it is mounted, or else firing it imparts a –4 penalty on attack rolls and the recoil knocks the wielder prone. A Large or larger creature can fire a double hackbut one size smaller than it is without its mounting as a normal two-handed weapon and without the danger of being knocked prone, but takes the normal penalty for firing an inappropriately sized weapon.
(I would, personally, but maybe that is just me. Take a -2 penalty, do 2d10 points of damage base, as a two handed side arm. Good with a minimal investment of feats in deadly aim and vital strike, bad to rely on if you aren't a musket master. As the damage doesn't scale well when compared to a two-hander barbarian or fighter with a viscous weapon.)
Going by RAW, I don't see why you shouldn't allow this. It seems logical to me that this works the exact same way one step further down the "scale".
Not sure if I like the ruling for large characters in the first place though...
Who cares about being prone ;)? Take a large one!
Better yet lay down then fire it!