I'll refer you all to
this webpage.
If you keep the flow speed under 5 fps (feet per second), then everything should be fine.
There really isn't such a thing as too much flow. There is a point where the heat induced by the flow becomes a significant portion of the total heat load on a rig, but we're not there yet.
If your tubing expands because of the pressure caused by the flow, then you might be using tubing that is not meant for this kind of flow rate. If you can't find the specifications for your tubing, then I'd recommend upgrading to vinyl tubing (braided optional).
I'll be testing my pump (Little Giant 2-MDQ-SC) this weekend, with 3/4 inch ID braided tubing.