Replying to Altitude affecting boot fit?

This is gonna sound totally unhinged but does anyone else notice a big difference in how snug their boots are at different altitudes?

I bought my current boots a few years ago when I was still living at 7200' in CO. Fit perfectly, super comfortable. But now that I'm skiing at places that are much closer to sea level, it feels like my feet are swimming in my boots. I'm pretty sure I'm not making this up because even when I go back to elevation for breaks, they fit tight again.

Anyone know why this might be happening? Or what I could do to get them to fit at both altitudes?