Thank you for the feedback, all. That chart is especially handy.
To add a little more detail, I plan to use it on hikes as well. I’d like to fit a cell phone and keys in there as well as the handgun at a minimum. A small, thin fly box or some odds and ends (match case, water tablets, light) when hiking would be great if the size of the kit bag doesn’t get obnoxious.
I am assuming from the thread that the runner’s kit bag is basically handgun, phone, keys, and that’s it. I think it would be wise of me to go one step thicker but not to the highest level of thickness.