As far as I recall, weapons that are tagged as Heavy penalise squaddies who move with an accuracy penality. This is quite appropriate - heavy weapons operate best when they are at rest. But players respond best when they are both rewarded for appropriate behaviour as well as when discouraged against inappropriate behaviour, so why not dangle a few carrots as well as wave a stick?
For example:
Add a small bonus to reaction fire when the isHeavy conditions are met - the squaddie is clearly preparing himself for targets of opportunity.
Make it slightly cheaper to fire aimed shots - the squaddie has got himself in the best position he can when not in motion.
Make it slightly more expensive to rise from a kneeling state - this represents the additional bulk and weight of the weapon.
These don't have to be big things, just little positives or negatives that reflect the style the weapon requires.