Well you probably have seen once pictures that people like to make when they are visiting the Eiffel Tower or the Tower of Pisa where they aim their camera from such a point of view that it appears they are physically pushing the tower or that they appear larger than the tower. It's just the result of playing with the camera angle and the perspective. Well Oda pushes this to the extreme in his drawing style. Yes he provides even data on how large specific persons actually are and many are huge, but he toys around a lot with the perspective of these characters when he wants to emphasize something.
For example take now that scene posted in this thread of Akainu, Shanks and Coby. This wasn't just about Akainu and Shanks being tall guys, Oda wanted to emphasize in what kind of messed up situation Coby actually was. He was trapped between an admiral and a Yonkou, these are people who are really 'great', not just in stature, but also in a more metaphorical way. To be blunt Coby is like a tiny ant between two titans that are clashing and Oda wants to visually give you that impression. With other words this is a drawing technique to convey a message without words.
It's the same thing with the example of Whitebeard. In the first picture he was just a crewmate among many under a pirate captain, in the 2nd he is the mighty Yonkou Whitebeard, the one closest to the One Piece and the one who could go toe to toe with the former Pirate King. In the first picture he pretty much is a different person than he was in the 2nd one.
If you're going to approach this in the way of 'he's screwing things up with, it makes no sense', well then you are completely missing the point and are being superficial. Physically it's not really correct what he does, but it's not like he's doing it randomly, there's a purpose behind it.
