This is a charade - where two or more parts are joined together to form the answer word.
You need to find a word meaning 'boxer' (think about what a boxer contains) plus a word for 'heavyweight' (as in weight measurement). The phrase 'could one say' suggests the boxing connection might be whimsical or indirect.
Answer: CHESTERTON | CHESTER (a 'boxer' in the sense that a chest/box contains things) + TON (heavyweight unit). The phrase 'could one say' signals the playful connection between boxer and Chester.