Why is the stability of (Z)-cyclodecene 2 greater than that of (E)-cyclodecene 1?

I'd think that since 1 contains a trans double bond instead of a cis double bond, it should experience less steric repulsion and hence be more stable.
However, I've read that the cis isomer 2 is more stable than the trans isomer 1 if the ring size is 11 or smaller. For ring sizes larger than 11, the opposite is true.
Why is this so? Also, what would happen if there were more double bonds?