You may want to take another look at your tires; they usually have a pressure range, rather than just one number on them. For example my hybrid's tires (Bell brand from Wal-mart; 26 x 1.75) state 40 to 65 psi.
When I started on that bike 2 years ago, I was about 400lbs; now about 330lbs. Never got the tires all the way to flat with me on it
though I did break spokes - and now I build my own wheels.