Could be you just don't like all that extra weight hanging on your belt.
Picture it this way. Take a 3 lb weight in your hand and hold it straight out from the body. 3 lbs isn't much, but after a short period of time it gets quite heavy and your arm will start to fatigue. Now take that same 3 lb weight and with your arm straight out from the body, push the weight up against a wall and hold it there. The harder you push against the wall the more the wall will help support the weight. If you push hard enough you could probably hold the weight up all day.
Now picture your belt as being the arm that is holding up the weight, the weight being your gun and your body being the wall it is pushed up against. The tighter you wear the belt with the gun clamped between it and the body (as with IWB carry), the less weight the belt has to support. Outside the belt, the belt supports 100% of the load.
I really don't know nothing about physics, but that's my theory and I'm sticking with it.
T