Question one: why do stars change very little while they are in the main sequence stage? question two: why does a main sequence star eventually grows in size to become a red giant?

Q1 As the main sequence star glows, hydrogen in its core is converted into helium by nuclear fusion. When the hydrogen supply in the core begins to run out, and the star is no longer generating heat by nuclear fusion, the core becomes unstable and contracts.

Q2When stars first begin to fuse hydrogen to helium, they lie on the zero-age main sequence. ... The core of a red giant is contracting, but the outer layers are expanding as a result of hydrogen fusion in a shell outside the core. The star gets larger, redder, and more luminous as it expands and cools.