I am both a big man and a fan of IWB carry. Unfortunately I have found that with some large frame autos, a belt slide holster is more comfortable, and with some pants that don't fit me as well as they used to this is the only option.
I could, of course, go out and buy larger pants, but I like to delude myself into believing that I have not gotten larger over the years.
Oh yeah, holsters. If you MUST have a retaining strap on your IWB holster make sure that the portion of the strap that "breaks free" up against your body is reinforced or stiffened. Otherwise what you will find is that there is too much give in the holster once it is broken in. Instead of a nice crisp release, the softer leathers will want to flex into your abdomen a bit before release, which considerably slows your draw.
I use a basic leather IWB holster, thinner is better in my opinion for an IWB rig, so what I have done is cut off the strap on both of my leather IWB holsters. The gun remains quite secure even without the strap. They have both passed the "BOWLING TEST" and the "RUNNING AFTER THE DOG TEST" with flying colors. That is I can both bowl or run after my escaping dog without having to worry about the gun falling out!
One other point, based on your initial post, with some pistols the weight of the gun will almost require you to wear a belt, even with an IWB rig. Otherwise you are going to want to keep pulling your pants up all the time.
These are just some points I have learned over the years, YMMV.