At my cabin I use a 12 volt Flojet pump to pump water from the lake to the cabin. I am using 1/2 inch plastic pipe for the intake and outlet on the pump. Would I see an increase in water flow if I increase the size of the pipe going from the lake to the pump to 3/4 plastic

Maybe... it depends on the total flow, and if the current pipe is restricting that flow.

You certainly won't see a decrease in flow!