First all understand that all the clamping happens at the collar and nowhere else. You also need 3" or so or snug running contact below that to stabilize the post so it can't rock which would cause creaking. Anny additional contact beyond those 3" is meaningless, so put the issue of the bulge formed tube out of your mind.

So assuming the post fits, which I'll take at face valus, let's look at the clamp. Take a look at these two clamps. Focus your attention on the transition area between the ears and band. No.1 No.2

Notice how the first is really beefy there and the second isn't. With the second, if tightened hard, the ears will begin to bend, while with the first all of the force of the bolt goes to tension the band. So before you go farther, is your collar like the first or second. If not, buy a better, beefier collar.

Now to get the best clamping, you want to grease the collar so it can slide on the tube and clamp evenly. Then because you have a well designed collar, you can feel free to use some added torque on the bolt (throw away the torque wench and tighten this until you feel the bolt resistance ramp up). Assuming it's a stainless steel bolt, try to stay below 7ft#s.

Now look at the rest of the picture, you have a polished and anodized post which is fairly slippery, then you grease it well to make sure it's slippery, then you want to hold it tightly. Doesn't make sense does it? But you do need antisieze or grease, so here's a trick. The clamp only holds at the top so you can grease everywere but there.

Put a piece of tape around the post at the right height. Now use solvent to thoroughly wash all the grease from the post, leaving clean dry bare metal. Use a rag dampened with solvent to clean the inside of the tube down about 1/2" or so, leaving grease below that. If you want you can add more grease below with a brush being careful to keave the top 1/2" clean. Insert the post, turning it around to spread the grease as you feed it in to the desired depth, but do not go oo far and pull it back up. This will give you a dry area for best clamping and should solve the problem.

If not, start fresh the same way, but paint a 1/2" band below the tape mark with nmedium grit lapping compound, which will bite into the frame an post locking them together. Save this in case nothing else works, because it will mar the post, and can score the metal if you twist the post while it's tight.