And half a dozen ferengi could take control over the flagship of Starfleet.

For me, its quite simple: Its all about crew quality in combat. The Enterprise for example packs 1000 men, women and children that cant even hit a target 10m away with auto-aim assist phasers. The ISD however have 10,000 highly trained Stormtroopers with body armor.

But then again, they have no way of beaming them over. But to be honest I dont think the beaming ability help the others very much, Stormtroopers dont walk around slowly ignoring everyone that isnt a threat.