I'm going to agree AND disagree with you here, because while from the outset that appears to be nonsense, there is definitely some other aspect of the physics of the Mag return pump to take into consideration, so they would not have made such a statement.
Why don't you call Danner and ask them. I couldn't get through to a tech person, but the person I did talk to said they had specifically had this discussion many, many times and the knew the answer right away, so it is a common question that they get.
I see where you are coming from, because as an engineer,I have MEs in my office that told me that your pipe flow rate is dependent on the smallest diameter pipe in the run. So it would make common sense to say the pump nozzle determines the hose size. However the pump output point may have slightly different properties because of backpressure requirements on the impeller and that pressure release after exiting the nozzle. If that pressure isn't eased, you get too much backpressure and your flow decreases.
Whatever the case may be, the answer, although not immediately logical, is clear from both manufacturer recommendation and from field experience - the pump output return hose needs to be 2x the ID of the output nozzle. Period.
It's just one of those things that you are going to have to prove to yourself by performing an experiment, or you are just going to have to accept it as fact and move on. Either way, it is what it is, and just stating that it 'cannot be true' or that it 'just doesn't make sense' does express your confusion/frustration - and believe me I share that with you - it doesn't matter unless you prove them wrong. And my guess is that you would find their statement to be correct or else why would they A) put it in their manual B) put it on the box C) tell people the same thing when they call in (it was their most common call, by the way). This is the conclusion I reached.
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