Most of that bounding box is emptyness that doesn't count for collision detections. The bounding box only matters for what landing pad a ship fits on, nothing else. A big (by volume and mass) ship doesn't need to have the biggest bounding box, and a small ship can fill most of the maximum bounding box possible.So the box of the Beluga (the largest so far in the game) is 209.10m x 131.60m x 38.6m, which makes the box of the Panther merely 3/4 of that size.
Fdev felt that 1200 tons of cargo is the right amount (whether the playerbase agrees or not) and the PC needs to be the size that it is to fit that.