Well, the SMG/pistol bullets are wider, aren't they?

actually no they are not.

Well last i checked 9mm was wider than 6.5mm.

But as far as weight goes, there are many different types of ammo in arma and i think a some of it is caseless too which is lighter. So perhaps this is intentional.

I think it's possible to carry more SMG rounds then rifle rounds because it is lighter regardless of what size they are, the game bases it on wight not and size, and that would mean that smg are heaver then rifle rounds witch would not be right at all.

Thats not strictly true, a .45 round weighs much more than say a 5.56, you'll have to be more specific with which magazines your referring to.

Pistol mags don't take more space than rifle mags in the game. At least not in dev branch. Sub machine guns mags seem to take more space though I think they should take as much space with rifle mags. 9mm mag can be longer with same amount of rounds but it's less wider than 6.5mm. I'm not sure are the capacities of the vests etc. only measured in weight or do they take account the size of an object. If they take account the size then this could be fixed but if weight then I don't know.