See the table I linked above. It's not entirely accurate for comparing expectations, but its crystal clear on force. There is also a calculator at the top of this page under "app".
The velocity of the gun (FPS) and the weight of the BB (grams) are used to work out the muzzle force (joules) of the projectile.
Simply put, the FPS limits are set by choosing desirable SAFE muzzle force.
So:
If you chronograph a gun with 0.2g ammo you will get an FPS reading which will tell you the muzzle force in joules. If you increase the weight of the ammo the FPS reading will go down, but the muzzle force in joules will remain similar.
If you increase the power of the weapon, the FPS goes up, the joules go up. The weight of BB used is an irrelevant factor to making this point.
And most importantly when chronographed with 0.2g your gun will be considered unsafe on health and safety grounds - and if you don't get caught you will be (A) cheating, ( B ) dangerous, & ( C ) <expletive>.