^ Venators were intended primarily as carriers, not true battleships
Their standard armament was a lot weaker than even the smaller Victory class, with only 8 dual turbolaser batteries compared to the Victory's 8 quad batteries and the Imperial's octuple batteries
Also the hangar's a lot longer than 500 meters, it runs (essentially) the entire length of the vessel up to just past the bottom of the superstructure
That's closer to 700 meters given the Venator is a little over 1100 meters long in total and, at its widest, 548 meters wide (it averages probably closer to 250 meters)
We've seen in TCW that Venators are multi-level, having 2 separate hangar levels that can by cycled between with large central elevators
So considering the widest fighters, ARC-170s, are about 20 meters wide, and the longest, Y-Wings, are about 20 meters long, and those are the exceptions, not the rule, (most of the fighters are closer to 10 by 10, like V-19s and Eta-2s) I don't think it's inconceivable that a Venator, given a little bit of finagling, could stock a grand total of 420 fighters