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

considerably 是副词，由形容词 considerable（相当多的、相当大的）加上副词后缀 -ly 构成。considerable 来自动词 consider（考虑）加上形容词后缀 -able，原意是"值得考虑的"，引申为"相当多的"。 considerably 在这里表示程度，意思是"相当地、显著地"，强调变化幅度大。