Up until about a year ago I was 5'7" and 255 lbs. (I'm still 5'7" but I'm down to 198 as of this morning, still fat but not as fat.) Point is, I was / am FAT. So my experiences with IWB may have some valid comparison to what you are looking at.
Anywho, I have carried a SA Mil-Spec in a Milt Sparks Summer Special off and on for the better part of 10 years and it's been one of the more comfortable IWB hosters I've used. I have a VM2 for a Kimber Compact CDP that is also pretty comfortable, on a par with and possible just a tad more so than the SS. Of course, the CDP has an alloy frame and is a 4" vs. the steel frame and 5" of the Mil-Spec, but still...
In my experience, the biggest factors for a comfortable carry IWB are two-fold: get a good leather holster from a reputable manufacturer and get a great carry belt that matches the loops / slots of the holster. I can carry IWB with kydex , but leather beats kydex (holstex / polymer / whatever) hands down for comfort. And if it's from a reputable manufacturer, it will work well. I have gun belts from Sparks, AG Custom, Don Hume, Spec-Ops, Bianchi, and Daltech Force. The Spec-Ops belts are only good for IWB, the rest are good-to-go for any method you desire. My current favorite belts are from Daltech Force and are steel cored. Currently own two of them, one in black and one in distressed bison. They are rigid and pretty much lock in the holster so there is no outward bow which can / will cause your gun to show more. Real world story: tonight when the wife and I went out to eat I had a CZ 75 P-01 in a K-T Mech Akela holster (OWB) and a Bluelineconcealment double mag carrier with two 16 round mags in it. I wore a Hawaiian shirt one size oversize and would defy anyone to spot either the gun or the mag carrier. A good gun belt will do more to help concealment than a good holster will do. Get both (good belt and good holster) and you can have both comfort and concealment at the same time.