For example, if you've been working on a database system for 8 years, there aren't that many companies that need to develop their own internal database system so the matches are slimmer. Or if you've been doing SRE on 1k k8s clusters, there aren't that many companies that are doing k8s at that scale and working at lower scales is completely different.
Another thing with going from a mega corp to a startup/smaller business is the expected performance. Mega corps are slower with low acceptance of errors while smaller businesses are faster with a higher acceptance of errors.
This doesn't even factor in that it also seems like smaller companies are also doing downsizing at the moment. I'm seeing smaller companies add in policies which are clearly designed to make people want to quit to companies getting rid of all of their remote devs because they're the easiest to fire. The job market doesn't look good right now.