That's clearly a perspective shot. Nami is in the foreground and the "camera" is at a high angle so she looks the same size as the bee. The "giants" are shown from a low angle and are in the background so they look larger. But really they look about the same size in comparison. If those were real giants, the bee would just look like a normal bee compared to them. Same with the "giant rabbit" ear god, it would just be a regular rabbit to them if they were giants.