Now this I do not believe to be correct at all...
I am no expert at all but the larger the outlet plumbing, the more dramatic the head pressure becomes...
A pump would have to push up twice as much "water weight" with 1.5" plumbing than it would with 3/4" plumbing... You have twice the water weight, moving slower...
With some things, it is far wiser to believe, than to not believe. However, when there is a natural science behind it, belief is not necessary. But yes, in a way you should not: 1.5" is the minimum size...
I had never heard of such a thing, (such a large increase in pipe size being needed,) till it was put rather bluntly to me. I had always indicated it was prudent to increase the pipe size by one size. The mag drives are particuarly poorly engineered pumps...the flow increase is along the lines of several hundred gph.
I direct you to the Danner Mag 9.5 instructions, page 2 just below the bar graph:
http://www.dannermfg.com/Store/images/instructions/ZG100.pdf
Pressure and weight are not the same thing. Weight involves mass and the acceleration due to gravity, it is static, at rest. Water is a liquid, pressure is static or dynamic (in motion,) and though it does have mass, and is affected by acceleration due to gravity, that has nothing to do with head pressure. Folks very often get weight and pressure mixed up.
We have a 1" pipe, 100' tall. Filled all the way to the top with water. The pressure at the bottom of the pipe, (head pressure) is x. We move to the ocean, we go down 100', and we find that the pressure at that point is also x. Pressure at a given depth in a volumes of water, is equal, and it is equal in all directions, regardless of the size of the container, Not accounting for local ambient pressure. (Barometric pressure.) Though the weight of water in the ocean is far greater than the weight in our 1" pipe, at the same depth the pressure will be equal.
So from this, it can be seen that weight is not a factor (e.g. how much does the water in the pipe weigh.) Having a pump push water up 100' in 1" pipe, the pump will see the exact same head pressure as it would in the middle of the ocean at the same depth. However, the output of the pump will be greater in the open ocean, than it will be in the 1" pipe. The reason is friction loss, or loss in pressure due to being in contact with the pipe walls, in the 1" pipe. There are several terms for pressure in fluid dynamics, but none of them are related to weight.
Since this is dynamic, the natural science of this topic is called fluid dynamics, a subdiscipline of fluid mechanics, (deals with fluid flow) the natural science of liquids and gases in motion. More specifically it is Hydrodynamics...seperates it from Aerodynamics.
We are getting pretty far away from the topic in this thread, however, fluid dynamics is important in drain systems, as well as return systems.