Hancock (and Law) stand 6'3", which is unusually tall for a woman and decently tall for a man, but still tiny next to Crocodile, (8'3") Jinbe (9'10") Doflamingo, (10') Kuma (22'6") and Moria (22'7"). Mihawk is 6'5", and he also looks puny next to his cohorts.
I didn't worry about making them too scale in this, or I would have lost too much detail in order to fit the two demi-giants into the last frame.
Some One Piece characters just seem to be abnormally tall. Blackbeard and his main crew are all in the 10-foot range, and several of Franky's cronies in Water Seven were 20-footers. Considering that demonlike characters like Moria and Magellan (and even weirder guys like Fukuro) are accepted as human beings, I think that Oda just likes making some of his characters supernaturally big or weird-looking.