These words mean about the same thing, and it makes no difference whether one says "It was a relatively minor accident" or "It was a comparatively minor accident." What does make a difference is the use of either term when there is actually nothing to be related or compared to. Neither word can be justified in a sentence such as "There was relatively (or comparatively) little discussion of this brand-new proposal." "Little discussion" relative or comparable to what? Can any "little discussion" be typical? A "brand-new proposal" is unique and has not been discussed before, either at length or briefly. In short, relatively and comparatively are often meaningless words that add nothing.
Dictionary of problem words and expressions. Harry Shaw. 1975.