I assume you made the jump. What was the outcome? Did you find it just as easy to find offers elsewhere as you did when you were at a big company (if you were looking at all)?

I am partly -- perhaps irrationally -- concerned that going to a startup will somehow make me less attractive to companies for working at a smaller scale.

I'm sure you are wrong -- you'll not be less attractive, moreover, I think you'll be more attractive because you'll have had both working experiences: small and big companies. As for me I'm definitely not receiving fewer job offers now, after the jump.