A standard (50 gallon) water heater weighs about 500 lbs over an area of about 3 square feet = 166 lbs/square foot. My 300 weighs about 2800 lbs over an area of 16 square feet = 175 lbs/square foot. Not that different...
but it is very different. because the weight is not spread out over the entire bottom surface area of your tank.
it is only spread out over the part of your stand that actually contacts the floor.
so instead of the weight over a 36x18 area (for a standard 50g), for example, that 500 pounds might only be spread over a 1" perimeter of your stand, so it would be 108 square inches instead of 648 square inches (if it was full support bottom).
so it 4.6 pounds per square inch vs 0.7 pounds per square inch for example.
your 300g will exert a significantly greater pounds per square inch than your 50g will.
so we have to be careful when doing the math.
its easy to make errors in assumptions.