Can't say about the Fork Tube being too small but a lot of times the Clamp itself just needs some grease.
Get some white lithium grease and grease up the bolts on the clamp:
Most of all the clamps are made from Aluminum and the bolts are steel and since Aluminum is softer than steel the bolts tend to Bind up in the threads before actually tightening the clamp all the way.
It feels like it is tightening because it gets harder to tighten since it is binding in the threads.
If you don't have any grease at home in the geerage, grab some cooking oil from your Mom and put a drop or two on the bolt. It will work for now until you can pick up some White lithium grease. It is only a couple bucks at a hardware store or Auto parts store.
