I would say unless you live in a very warm climate or do a lot of sweating through activity (not just sports), that one gallon is more than enough even for a big guy.
A good guide is to guage how often you need to go to the toilet (though this can change depending on health problems etc..). You only need enough to stay well hydrated. When I was younger and a lot sportier (gym rat) I would consume enough to be going to the toilet every 2-3 hrs and that my urine was near enough see through, that was always ideal.