Not sure if this is a glitch or not, but when I added about 1000 Milky Way masses of atmosphere to Mars, it's profile said this:
And on top of that, the planet was frozen.
Also, the mass of the atmosphere in US2 doesn't seem to have any affect but being a greenhouse.
I'm sure it's on GA's
Naughty List dev list to have the atmosphere have an actual affect on the system physical wise, but just giving everyone a heads up.