As said, a lot of a modern combat aircraft is made up of the cockpit, the life support systems, the avionics, the engines and fuel tanks. A Viper in theory has a smaller life support system because it doesn't appear to have a pressurised cockpit, the pilot wearing a sealed suit instead, has no apparent anti-G system and appears to have simpler avionics (the weapons system alone must be very simple, given they just use guns and unguided rockets). The bulk of the Viper fuselage seems to be engines, but again, they're smaller because there is no drag to fight against in space, and the fuel tanks are smaller for the same reason. It's not unrealistic to have space fighters smaller than air fighters.
I believe Galactica was meant to have four Viper squadrons, two per pod, plus a Raptor squadron and a few shuttles. In this case, a full complement of 80 Vipers sounds reasonable, and would allow each fighter to be stowed in the entrance to the launch tube ready for launch under normal combat ops, where there would be less issues regarding supplies, crew, maintenance and so on. You don't over-stuff warships, because then the excess gets in the way and slows down operations. You also don't leave equipment and supplies cluttering gangways, where they pose a fire risk and can block access if they fall over, but the show abandons these practices.
