video size != costs! Most movie production costs are mostly humans.

Video size absolutely correlates with cost. It's not 1:1 but there's a relationship. The entire pipeline masters from uncompressed sources. When you need to color, crop, re-frame, or add VFX, you need to read/render those files at original resolution. Render time (and equipment that can do it) becomes a factor, particularly for post-production vendors, who pass on the cost (and mark it up).

True, Post Production is not the largest piece of the pie, but it still adds up to millions of dollars.

Only uncompressed. Especially when there's not a ton of motion.

Not necessarily 2.5x. Video compression is a thing.

presumably they're storing the raw, uncompressed video? at the same time, I would imagine storing a couple extra terabytes of data isn't a significant cost in the scheme of a production.