OK. Maybe I did not say that correct and I did not state the obvious that a pressure difference is required. The bigger the pipe, the lower the head loss, the higher flow rate you will get and at a lower velocity.
If you always have two 1.5" suction lines running, then up-sizing them will likely make little difference. If you only have one 1.5" line, then increasing the pipe to 2" might be more noticeable.
My pool used to only have one 1.5" suction line from the skimmer (drain connected to skimmer). I happened to find an abandoned suction line from a hole in the wall (I think used to be for a waterfall pump) and have now plumbed that wall suction line (also 1.5") together through a 3-way valve which then up-sizes to a 2" line into the pump. On high speed, if I have both suction lines open, I can see a small pressure increase in the filter, so I am getting a higher flow rate which helps for my solar. I may add a flow meter to get some hard numbers out of curiosity.
When I am not running solar (like now) and mostly running on low speed, I actually close the wall suction line so that I get higher flow through the skimmer on low pump speed. I might actually try to automate that valve to turn in conjunction with my solar valve to get the best skimming I can on low speed.