Small Ammo Kit makes no sense to choose, because the Heavy wins every time, it's a straight upgrade of the two.
But what if it didn't have to be this way?
Small Ammo Kit features:
- left click to replenish own ammo, right click to replenish other people's ammo (only primary and secondary)
- faster draw speed than Heavy
- can't be dropped
Heavy Ammo Kit features:
- its current ones
- an additional pool of points to resupply helmets, up to one single Exo