I'm 40 and my ribcage has definitely expanded over the last 10 years. I've never been pregnant and I'm the same healthy BMI weight (possibly a bit lighter) than I was in my late 20s/early 30s, but I have bras and dresses which no longer fit because they are too tight around my ribcage - and it's definitely not fat!
I've googled but all I found was that it happened during pregnancy. Anyone lnow why else it might happen? It's slightly annoying because I have very small boobs and I'm also very slim generally, but I now have quite a big chest measurement which means some dresses and tops which fit elsewhere are too tight across my chest!