Why are old fashioned wooden barrels made with a bulge at the waist?

Question

It would seem less trouble to make and take up less storage space if they were made with straight sides.

Answer 1

The curve makes handling of the heavy barrels (when full)possible.

The barrel shape makes it easier to tip the barrel over from a vertical to a horizontal position for rolling it to a new location. Because the curve acts like a rocker. (try tipping a straight sided container on it's side without damaging it) and it makes it easier to push the barrel into an upright position after rolling it to a new location. The curvature also makes it possible to change the direction of roll very easily, which is all but impossible to do with a straight sided container.
A barrel shaped object always seems easier to roll once on it's side, than a straight sided container is, also.

Answer 2

The pieces of wood are straight. When the barrels are made, they are forced into a curve. This helps keep the barrels tight. Pressure from the inside won't cause the sides to expand.