Maybe it's about the perspective you're given from your fighter.
The fighters are pretty big, i mean bigger than f-16's and stuff.
One thing i don't see is how an orion is supposed to hold 10000 people.
I mean, star trek's enterprise is more reasonable with having 1000 people.
If an orion had that many aboard it'd make more sense, especially for a hecate.
As far as a how big a fenris is actually supposed to be, just look at the size of the windows, the place where the dish is like the command center on it.
But, yeah, the fighters are pretty big, they're no where near compact at all.