The huge atmospheric pressure variation (25-30% of the nominal 6.7 mBar) equates to 0.5 to 1 m of CO2 snow over a disc 1500 km across - the seasonal polecap. SO in fact, it doesn't take much CO2 snow/ice to make that big pressure change.

(on Earth, if you froze out the atmosphere, you'd have a layer 10m thick over the entire planet - 1 Bar of pressure)

Now, let's see how much CO2 is in the southern polecap in summer:

The permanent polecap is less than 500 km across, so if it has 20 m of CO2 ice, that corresponds to 4-5 mBar of CO2.

The implication of this is that as you go to a warmer mars in higher obliquity cycles, once you hit 11-12 mBar, that's the end of the road. No more global warming.

Unless there's more CO2 hidden somewhere...