Solo in 1P vs 2P tent in very heavy rain?

The heat output from a person will warm up the smaller volume more.  The warm air can hold more moisture.

I think the bigger tent is also better for sanity if you are trapped inside with rain for an extended period. I’d also agree that you can also put more distance points between yourself and a wet fly sheet if you have even a moderately bigger tent.

The larger the tent the better in long cold rainstorms—within reason.  Why?  Because I don’t want any part of my fully lofted down sleeping bag touching any part of the condensated wet inner tent walls, especially at the foot end.

I pulled a 19 day trip with my Akto tent in many long cold April rainstorms and dangit that thing was way too small for all my winter geese items.  Every part of my subzero rated down bag touched the wet inner tent.

As far as hunkering in for 3 days during a nonstop blizzard—the bigger the better.  Everything must be stored inside the tent except for my hiking pole and snow shovel—I leave my food bags in the tent vestibule.

I leave my food bags in the tent vestibule.
We bring all our gear into the inner tent, and pack a lot of it between the air mats and the sidewalls. Keeps our quilts off the sidewalls.

Cheers

Ben’s point is very good. The air in the smaller tent will be warmer and hold more moisture. When the warmer air meets the cold fabric, you will get more condensation. Assuming you can keep the rain outside the tent, the more ventilation you can mange the better.