adverb
greatly; significantly; on a large scale; widely
Adverbial form of 大きい, used to describe doing something in a big, extensive, or grand manner. Often appears with verbs like 変わる, 広げる, 見せる, etc.
彼の考え方は大きく変わった。
His way of thinking changed greatly.
事業を大きく広げる計画だ。
It's a plan to expand the business on a large scale.