It seems to feel innately fair to children?
It never felt fair to me, not one jot. In fact, I think I still resent having had the smaller room as a kid, despite having left home 25+ years ago. It really upset me at the time.
What I really struggle to understand are all the comments along the lines of the eldest needing a bigger room for sleepovers, or a desk for doing homework etc. And the suggestions that the youngest can have the bigger room when the oldest one leaves home.
Do you all have really massive gaps between your children or something, so that they don't need these things at the same time? Will the younger one not want sleepovers in a couple of years as well? Won't they start getting homework too? Are you going to move then, or is the younger one just going to have to suck it up because the older one got there first?
And when will the older DC move out so that the younger one can benefit from the extra space? My DSis didn't vacate her room until well after I'd left home.
Am I bitter about this? Yes, I am - it still feels really unfair, all these years later. Sometimes I'm so glad I have an only child!
In your situation, OP, I'd toss a coin, or make them swap every couple of years.