>>11477911
Blackbeard is specifically designed to be twice Luffys height. It's not a coincidence that he's 344 to Luffys original 172. Luffy is in the foreground in that panel. By using perspective Oda can convey 2 characters emotions through facial expressions at the same time without having to use split panels. He uses that technique a lot, especially during fights.
It's no different to using it in photography to make characters look bigger/smaller.