Hmm, this is difficult to explain. かなり or だいぶ is used when the thing is not entirely different but far from your expectation or your thought. かなり and だいぶ are the same meaning. I think けっこう means "to a great extent." I wish this could help you.

kekkou means healthy kanari daibu means pretty much sorede...sono bunshou no imi wa PRETTY MUCH HEALTHY