Bigger rings are more efficient?

I was going to say that trackies tend to go smaller for events where you need to spin up to speed quickly and go bigger for the longer events with less speed changes. Allegedly the ratio of crank length to chainring diameter gives you more leverage with a smaller chainring, although I'm not sure if that's actually based on phsics!

I thought it was B*llocks, so I spend a couple of hours working through the sums and it seems to be true.