If your loadout is lighter then the rest will be in the ammo bag. Lets say one ammo is worth 100g.
Rifleman: 4kg rifle + 7x0.5kg mags + 2x0.3kg grenade = 8kg. This gives them 20-8kg = 120 ammo in bag and a magazine is 5 ammo while a grenade is 3 ammo.
With LAT having the same damage for all caliber the weight need to be normalised to e.g. 80mm. M72 weight 3kg but is only 66mm so scale it up to 4kg and pretend it's 80mm. AT4 is 84mm and weight 7kg but say 6.5kg if it was 80mm. 40 ammo for M72 and 65 ammo for AT4.
A LAT loadout could be 8kg rifle + 3 M72 or 2 AT4 for a total weight of 20kg. RPG loadout should count launcher and rocket separately.
There could even be pistol roles where the pistol and ammo is only 1.5kg and you get 18.5kg of HAT equipment.
As bandages are magical in squad the just put some imaginary balanced weight like 2kg each or something.
Insurgents could get a blessing for righteous fight and carry 25kg for balance.