Matmg = psA
where g is the surface gravity and A is the planet's area. (This assumes g is constant throughout the atmosphere, which is a good approximation for these purposes.) The planetary data is here and here.
To my surprise I found that the mass of Venus's atmosphere is 92 times Earth's, the same ratio as the surface pressures. Why would that be?
Since g=GM/R2 and A=4πR2, this means the ratio
must be nearly the same for both Venus and Earth (M=planet mass, R=planet radius). And they are, to about 0.1%.
Why? There's no reason I can see; they just are.
Am I missing something obvious? It's late and I've been working all day....
(By the way, this isn't true for Mercury or Mars.)