[Imp]

Its more than height its surface area.
Modern Russian MBT like T-90 are smaller than Western conterparts, Height, width & length hence a smaller target.
By comparison a jeep type vehicle is a small vehicle in all respects.
Amphibious landing vehicles like US marine LVT are large slab sided targets.
To determine size you need to take the profile into account, mix of silhouette for front & side as the game does not change size for the facing.