I agree, my example was of a typical squad, but the ammo loadouts are based on the individuals personal chioce and the mission plan.
For example the extra ammo would be ditched in favour of explosives if the mission was to destroy a target. And extra ammo would be carried if there was high probability of contacts with multiple enemy infantry forces.
Things like radio and batteries, medikits etc are shared between the specialists and the rest of the squad - this is in case the specialist steps on a landmine or is blown to bits (gorey but true) not all the kit goes up with him.
It would be a shame, but the ammo spaces are probably hardcoded, and could only be changed in a patch by bis.
The idea of a removeable bergen would solve this problem to a certain extent, and would probably be the best we will get.