iTunes uhd movies are not that big. More like 20g.A movie .mkv file (not currently supported) can be almost 100 GB in size. Likely a problem with a current maxed out 128 GB Apple TV.
And the hope is that Apple simply treats purchases of uhd movies like hd movies. I can download my purchases to my laptop or iPad. And watch it on the device or on the AppleTV using local media sharing. This avoids issues with internet outages or being on an airplane, crappy WiFi, expensive cellular. But we can only do this in hd.
I would like to be able to download my purchases to my computer or iPad and watch them offline in 4k hdr with lossless audio, and watch locally over the network on my appletv.