Pipe size only effects how much water is moved, doesn't hurt the pump to have a smaller pipe.
Pipe size also effects how much electric the pump uses.
Those things can all be calculated but here are some ballpark numbers assuming a length of 10' and PVC pipe.
Pipe size Head Loss PSI
1.25".........-22ft.............9.6
1.5"...........-4.3..............1.9
2"..............-1.3..............0.6
3".............-0.2...............0.1
4".............-0.1...............0.0
As you can see the PSI (pounds per square inch) of water inside the pipe drops as the pipe size gets bigger. For a pond we don't need water pressure, we only care about the amount of water moved. Just like your garden water hose, when you reduce the volume of water at the nozzle the water squirts out further.
What this means is, again ballpark, is your 7600 GPH pump would only move about one half to one third as much water with a 1.25" pipe as 2" or 3" pipe. Or you can look at it as having to pay twice as much for the cost of the pump and electric to move the same amount of water compared to buying a smaller pump and using the same size pipe.
In general, the fitting on a pump's discharge should be stepped up. There are complex technical reasons why the discharge on a pump is smaller than the pipe should be (pump needs higher pressure to prevent back flow), but the faster you scale up the pipe size the faster you reduce pressure in the pipe and the more water can move.
So a pump with a 2" discharge should have a 2" x 3" Reducing Male Adapter, MPT x Slip screwed into the pump discharge. Then you can glue 3" PVC pipe to the added fitting.
However, like I said at the beginning, you can use any size pipe and adapter you like, the pump will move some water.
