Hi,
The amount of fuel pressure delivered by the fuel pump does not matter to the gas tank. All the tank sees is that a volume of fuel is being removed and if air doesn't fill in then a reduction (vacuum) in air pressure will occur. As stated earlier, the pressure outside the tank at sea level is about 14.7 lbs per sq. inch. The difference in air pressure between the inside and the outside of the tank is what squished it.