Used to indicate that something is more or less than expected when compared to a standard or other things. Often translates to 'for a ...' or 'considering ...'.
See also: 割合
This winter is comparatively warm.
This apartment is far from the station, but it's relatively cheap.
The exam was relatively easy.
Even though it was my first time, I did comparatively well.
比較的 is a more formal, objective adverb used in written or analytical contexts, while 割合に is slightly more colloquial and subjective.
けっこう is a casual spoken adverb meaning 'quite/fairly', while 割合に more specifically implies a comparison against an expectation or standard.
Adverbial form of the noun 割合 (わりあい, 'ratio, proportion'), with the particle に. The comparative sense developed from the idea of 'in proportion to'.