Board Thread:Metal Gear Solid V/@comment-109.193.182.64-20150922080055/@comment-1504072-20150922094327

Since Metal Gear has always originated with the REX-like Granin designs from MGS3 (as far as current canon is concerned), I believe that is the simple answer. It's likely that both the in-universe design and Kojima's fondness for REX are equal factors in its look in the game. As for the sphere, its probably a radome as well, while the original MGS1 REX had a planar shape.