The video file for that movie stored on a UHD Blu-Ray would probably be in the neighborhood of 50-90GB, depending on various factors such as movie length, disc capacity (I think UHD discs come in two different capacities), compression rate used, and the inherent compressibility of the movie (stuff with a lot of grain, for example, will not compress as well).
A streaming service would probably compress that same 4K movie down to around 10-15GB tops, but probably even smaller, again depending on the same factors. In my opinion most people probably wouldn't notice a huge difference, if any, but it would definitely be noticeable if you knew what to look for (loss of fine detail in fast-moving scenes with a lot going on, a smooth gradient in a low-light scene on the blu-ray might have noticeable banding on the streamed version, and other compression artifacts like that).
Blu-ray torrents could fall anywhere on the spectrum depending on how it was compressed by whoever is distributing the torrent. If you see one labeled "remux" that usually refers to a video file taken directly from the Blu-ray without any additional compression, so watching that one would give you the same experience as watching the blu-ray directly.
A streaming service would probably compress that same 4K movie down to around 10-15GB tops, but probably even smaller, again depending on the same factors. In my opinion most people probably wouldn't notice a huge difference, if any, but it would definitely be noticeable if you knew what to look for (loss of fine detail in fast-moving scenes with a lot going on, a smooth gradient in a low-light scene on the blu-ray might have noticeable banding on the streamed version, and other compression artifacts like that).
Blu-ray torrents could fall anywhere on the spectrum depending on how it was compressed by whoever is distributing the torrent. If you see one labeled "remux" that usually refers to a video file taken directly from the Blu-ray without any additional compression, so watching that one would give you the same experience as watching the blu-ray directly.