Why is it that dual-gun weapons (dual-425 autocannons for example) use large ammo? I would prefer if a gun is of a smaller caliber, it uses the smaller ammunition. To compensate for the smaller ammo's damage modifier, simply raise the turret's firing rate or damage multiplier to match current specs. This would enhance immersion and create a uniqueness among the lower tier guns.

Why dont the "dual" and "quads" not just use 2 (4 in quads) charges per shot? of the smaller calliber? THAT would be awesome!

Because it is a game and they took the users ease of use into account.
If it is a large turret it uses large ammo. Simple as that.

This game is complicated enough as it is.

One possibility is that CCP could place these turrets in their respective category. Dual cruiser turrets could stay in the medium category and fire their respective charge size, albeit slower.

http://www.youtube.com/watch?v=Ve1DCnMWVfA does that look slower to you? I'd say double the fire rate of the medium, same range, with BS-grade tracking.

Is that turret using cap? Turrets in EVE do, well most anyway. There are drawbacks to using the weapon in the video, namely slower reload time as opposed to a single barrel version. Sure, you can have a crew of three manning it but that ties up more manpower and all those hands getting in each others way. Then there's the increased potential for mechanical failure. It's also more costly, ammo isn't free. All of that is considered a trade-off for increased defense against an attacker.

EVE has mechanics in place for the sake of balance and there are drawbacks here as well.

well. this is really the least significant problem. Have you ever looked at the tracking formula? Why does a ship which orbits another ship has to track at all? Its broken by design (lol). A better tracking formula would probably even fix blasters and short range ships.

consider them autocannons.