Well, there are T-shirts, and there are T-shirts. Some are designed to fit closely, and some are designed to drape loosely. I don't really care much for most T-shirts for daily wear in a hot climate, finding that the addition of a collar allows for better ventilation. Other than drape, there is the thickness and texture of the fabric, and several companies offer shirts in fabrics which are quite cool, yet not thin. Patagonia, Royal Robbins, Columbia, and North Face come to mind, and offer shirts NOT made in China, for those who pay attention to such things.
Anyone who cannot conceal a Kahr-sized pistol under such shirts is really not trying hard enough, or is not wearing the pistol in a location where the fabric is draped away from the body. As mentioned by others, a sturdy belt is an absolute requirement, so the weapon does not flop outward, against the shirt. Milt Sparks, Kramer, and other custom makers offer really wonderful belt, but Galco can offer instant gratification.
If I can hide an S&W N-frame under a Hanes Beefy-T in the Texas heat, IWB with a G. WM. Davis double-thickness belt, as I could when I weighed about 160 pounds at 6' tall, anyone should be able to hide a Kahr. For a while, I was too broke to afford a medium-sized carry gun, and wanted more than my J-frame when I went into town, so I toted my duty M58 duty sixgun. It wore fairly compact grips panels at the time, but an N-frame dwarfs a Kahr, regardless. I did conceal a Kahr K9 for a while, in a Sparks Summer Special, so I know.