I think you might be confusing size and mass.

A star (excluding systems with accretion) remains a relatively consistent mass for it's lifespan, but can increase in volume as it ages. This makes the older stars less dense and (for lack of a better term) more "wispy".

Stars with high masses will emit higher energy wavelengths closer to blue, while less dense red giants and supergiant's will emit more low engery wavelengths