Think of it this way -- you need people to man the turbolasers, pilots to man the ship's full complement of starfighters, engineers, navigations officers, communications officers, troopers en masse (imagine if you need to send a boarding party, or a detachment of troops down to a planet). If you have any ground-based vehicles they need pilots. Plus there's always people to man the mess hall.
If you really think about it, 1000 crew members is not hard to imagine at all
. You just have to remember it's not 1000 random people, it's 1000 people who all have jobs. For instance an Imperial-class Star Destroyer's complement of TIE fighters was one wing. That's 6 squadrons -- 72 pilots right there. Say maybe 50 turbolasers, 20 ion cannons, and now we're up to about 150 personel. Then if you wanted enough stormtroopers for a standard garrison deployment that's about 800 men. We're almost at 1000 already, and we haven't even covered the real