If you look into how an airlift pump works you can see why certain things happen.

Running a really massive pump on a small tube doesn't lift water because too much air will clear the pipe of water, and give the incoming air a straight shot up to the surface. It doesn't want to do any more work than necessary, so it doesn't bother to lift any water.

Think of it this way: if the air pressure is much higher than the water pressure at that depth, it will have the ability to push down harder than the water pushes up. A certain amount of that likely happens anyway or the water would try to go up the airline, but if it's too much stronger the water will have trouble getting around the air bubbles so that it can be lifted by those bubbles.

How it works:
Basically adding air to water reduces its density. When the air/water inside the lift pipe is less dense than the water outside the pipe, the water pressure at the bottom of the pipe tries to force water inward and upward to equalize the pressure. The air/water mixture literally floats on top of the denser water and thus can be raised above the surface of that water.

Conversely, things that normally float in water can actually be made to sink if enough air is forced into that water. (They've demonstrated that ships can be sunk if an underwater vent were to release sufficient quantities of gas into the water under the ship. It's one of the theories behind ships disappearing in the Bermuda Triangle.)

Point being that you've got to have the right mixture of air and water. Too much air and the water won't be able to "climb" on top of individual bubbles to be lifted up. Way too much air and the air will actually be forced downward to escape both up the pipe and out the bottom. And if you have too much water there won't be enough lifting force to make it come out the top.


Changing the diameter of the pipe (making it larger) means you need more air to lift the water, and that more water can be lifted. Bubble size also plays a role: if your bubble size is the same as the pipe's inside diameter, you'll lift water no matter what. Even at very low air pressure differentials you'll lift water, but it will go slow.

Also, regardless of the amount of air, water, or pipe size, an airlift pump can only lift water to a height of one-half the length of the submerged pipe.