A larger shirt not covering your sidearm, as well as the same shirt in a smaller size, does not make any sense.
The above suggestion of possibly using an IWB holster seems like it would help.
I carry IWB almost exclusively, and when I wear some of my larger shirts, I button the first one or two buttons from the top. This keeps the shirt from blowing open, but does not prevent drawing the weapon.
Just a thought.
