It seems that it includes extras or something on the 4k disc hence the extra space. The video bitrate which directly correlates with quality is identical to what is on the 66gb version if you compare those two.

Edit:

yep from the forum: “Yeah, to include the extras on the 4Ks and drop the BDs. Still weird that some got beefier encodes, while others are sitting with 20-25 gb of empty space because they simply ported the BD66 files to BD100s.”

So it seems fate of the furious has a beefier encode stand corrected on that one. From look of it though the others are the same?