Hydrostatic pressure is dependent on the height of the water only and not at all on the total volume of water in the tank. It's a little difficult to get your head around but maybe you can think of it like this. The pressure on the sides of the tank is just the pressure to keep the water "up" (or in it's current shape) not the weight of the water due to gravity.
Or try this thought experiment, when you jump in a body of water, are you instantly crushed if it's a very large body of water? No, of course not, the pressure of the water is dependent on your depth, not whether you're in a swimming pool or an ocean.
