He's considerably fatter than he was when I knew him.

considerably 是形容词 considerable 的副词形式，由 consider（考虑）加后缀 -able（能够...的）构成形容词，再加 -ly 变成副词。considerable 本意是"值得考虑的"，引申为"相当大/多的"。 considerably 作为副词，表示"相当地、在很大程度上"，用于强调程度或数量的显著变化。