I know...that's a weird one to grasp.

Suppose you have 1psi (pound per square inch) of pressure (sorry for the English measurements) inside the hull. You also have two tubes of different size with water in them. The amount of force pushing the water up the tube is calculated by multiplying the pressure (psi) by the cross-sectional area of the inside of the tube (the exposed surface of the water that the pressure would push on).

If you have a tube that has 1 square inch of area (cross-section), at 1 psi, you have 1 lb of force pushing up the water.

If you have a tube that has 2 square inches of cross-sectional area, it has it has exactly twice the amount of liquid in it - right? If you apply the same 1 psi to this tube, you have twice the area...again you multiply the area (2 square inches) by the pressure (1psi) and get 2 lbs of force...exactly twice the pressure exerted on the 1 square inch tube - and you have exactly twice the amount of water...same lift.