Aircraft weight

So I did a few calculations and tried to simulate a real aircraft fully loaded for a flight
I used the 737-700
Seats occupied-90%
Cargo - 80%
This only left enough weight to fill the fuel tanks half full
What if this was a 3.5 hour flight
So in the real world does the airline offer less available seats so they can fill up with more fuel
Or are the weights in IF unrealistic

I think that the cargo hold on a 737 would be mainly luggage rather than freight and so might be 80% full but not 80% of weight.