I dont think it will. The flow will still be dependent on the smallest diameter pipe.
If I understand what your asking.
All the plumbing could be 4" if you put a 2" section anywhere it will restrict the flow to how much the 2" section can handle not the 4"
I'm not over sure if this is exactly the case in all cases depending on the length of the run. For practical purposes in a typical aquarium situation it probably is.
True story here, we lived in Hawaii and where we lived it was common to run water pipes above ground due to rock content of the ground and difficulty trenching for pipe. Pipes were typically half inch galvanized pipe but over time they'd either corrode or get damaged and would be replaced with half inch pvc.
Problem with that was it would get brittle over time and either a rock or avocado (we had legit 2 pounders) would fall on it or a wild pig would step on it and the pipe would bust and need replacement. If you were below the road you could conceivably have a geyser at a partial break, if you were above the road the water would just ooze out and you'd have no water above that point.
We lived about 90 vertical feet above the road over a distance of 600 feet or so. One half of our pipe was galvanized and probably pretty corroded and between that and losing 14.7 # of pressure every 33 vertical feet we had pretty poor water pressure at the house. If someone was using a sink or taking a shower or flushing downstairs, taking a shower upstairs wasn't really working.
One day I wandered into an irrigation place to see if they had something stronger than the pvc that would hold up to abuse. He said driscoll pipe, then asked if I was above the road. Told him yes and he said to try 2 inch driscoll and replace as much of the old stuff as possible. He said our pressure wouldn't increase, but since there would be less friction restrictions over the length of pipe we replaced we'd notice more water flow/volume at the house.
I was rather dubious about the claim of more water at the house but since we wanted something stronger than pvc we picked up a couple hundred feet of the 2 inch driscoll pipe. After that we never had trouble with taking showers upstairs when something was going on downstairs again. We certainly removed some kind of restriction that allowed more water apparently, but the water pressure at the lowest faucet really didn't change.
We probably should have replaced the lower galvanized pipe as well, but it's a lot of work. I'm thinking that any restrictions will reduce your flow, but longer runs of restrictions will reduce it even further, so in a way the wider piping makes sense, but to what degree in a normal home aquarium situation it'd be noticeably better I think it'd be tough to tell.
