A gun is a gun. If it takes 5 space on a tiny ship, the same gun should take 5 space on a titan's hull.
What GC is missing is POWER PLANT. Bigger ships need bigger power plants to power all their stuff. The percentage of hull space must represent power plant, maintanence space (ie, crawlspaces for refitting/repair/maintenance), targetting mechanisms (turrets/battery/blister structure etc), and ammo storage/on demand production. That's the only thing I can think of that should cause a percentage used space for guns.
Now, in reality, if all tech is equal, and both tiny ships and large ships have the same guns/etc, then the big ship will win against a swarm. As long as there isn't any damage system. The reason is just as noted... your big ship will still be 100% effective at 99% damage. At 99% damage it's inflicted, most of your swarm has been destroyed (meaning less damage is being inflicted back on the big ship per round).
The question is one of fun. What would be more fun? Always wanting to build the biggest hull possible, or having a balance between hull sizes? I find that games in which the same amount of points invested in small hulls are competitive with large hulls to be more fun (ie, MoO1, where swarms of small ships were very effective against larger ships). MoO2 did not have that balance, so you always wanted to build the biggest hull you could (because 3 tiny ships in MoO2 could not kill the same logistal cost of 1 medium ship).
Now, if you have damage/critical hits in your combat system, that helps the swarm be more effective, or at least, on par with, larger hulls.