This has to be the correct answer. Different texture packs for ultra low, low, med, high and ultra high settings. Whereas consoles only need one of those texture packs
Could be worse, one of the previous Bethesda titles would download all language packs when using console compared to PC where you only got your native language.
You need special hardware to compensate for CPU bottenecks during real time decompression. There are a thousand ways PCs can handle that without a special chip - though your CPU absolutely uses special blocks to decompress popular compression algorithms.
The real truth is that the Xbox version isn't running with the maximum possible asset quality, so they don't need to actually bundle those assets in that version which in turn lowers the storage.