RDWC tubing

I'm going to be starting a grow with a friend soon and I told him the flow line should be twice as large as the return. He refuses to do it this way because he does not understand the reasoning and would rather have the same diameter all the way around. Why is the flow tubing larger diameter than the return?

I would imagine because the pump that is re-circulating the water is usually 1/2" or 3/4" line. Regardless of the diameter of the return line at some point it will be reduced to the size of the pump line.

Any drawback to doing 1" all around or is 2" better?

I ran 2" in my 6 bucket system and it worked fine. There is concern the roots can clog up the smaller diameter pipes is why it's suggested use 2" or larger.