I know what you are thinking now. By having a thicker pipe on one side than the other.... Imagine that you had two garden hoses connected together and hanging in a large U shaped loop. One garden hose is significantly larger than the other and they are full of water. The effect of gravity on the larger volume of water does not cause the water to shoot out of the smaller hose. They will seek the same level.
When the water in the hoses is put into motion (hoses looped together at the top), there won't be a pumping effect due to the differences of the hose diameters. This would be an example of a perpetual motion device.
Even though there are different volumes in each hose and the full hoses weigh different amounts, gravity affects each water molecule the same. In physics, energy equations are mass times gravity or velocity (et al). Objects are reduced to particles for simplicity in the equations. Solids are composed of many particles and since they are moving together in unison, they can be calculated as one. With fluids, it is impossible to represent all of the fluid as a single particle since it is really an accumulation of many particles moving differently in unison. In fluid dynamics mass is replaced with density (mass/volume) in the equations. This removes the commonly percieved problem of differing volumes in pipes and allows the 'particle' representation.