Pumps are often designed with restricted discharge nozzles for better efficiency at higher heads. You should not assume that is the correct line size for your return. The choice of line size is a compromise between pump power consumption and the cost of the piping. I have sized lines throughout my career as an engineer and I would not generally use a 3/4" pipe for more than about 300-400 gph. You can certainly get more flow than that with higher head, but you are consuming a lot of extra power due to the increased friction loss. The pressure loss goes up with the square of the velocity, so it ramps up quickly. Aquarium pumps are generally not very high head and cannot overcome restricted piping. It pains me to see people paying several hundred dollars for these DC pumps, and then using undersized piping which forces them to run at higher speeds, thereby negating much of the benefits of the pump. As a rough rule of thumb, I would use 3/4" up to 300 gph, 1" up to 600 gph, 1-1/4" up to 1200 gph, and 1-1/2" up to about 1800 gph. If you do this, you will keep your frictional losses to no more than about 2-4 feet of head in most applications. Add your elevation gain and choose your pump for the flow and total head. Be wary of any of these flow killing, high velocity, discharge nozzles. For an M1, you should be able to achieve up to about 1000-1200 gph at max speed in most systems, but you would need to use at least 1-1/4" pipe.
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