Paul I am guessing here, but the thinner the glass I would expect the less light absorption, and therfore less heat generated, Its a bit like when light is absorbed the deeper down you go into the sea. The light must change into heat.
While we are not talking about great changes in thickness, because the light is so intense a small difference in thickness may change its apsorption/heat properties.
Hence, thinner is more efficient at reflecting than thicker of the same material.