A reported size is different from an estimate
Some source formats include an explicit byte count; others offer only an approximate size or bitrate. An estimate based on bitrate also needs a valid duration. The result should not present a guessed number as a measured file size, and unknown is more honest than a misleading tiny 4K file.
Video and audio may arrive separately
A high-resolution video stream may contain picture only. When a compatible audio stream is available, the server combines them for the finished file. The displayed total must account for both streams; an audio-only size cannot stand in for the whole video. Actual output can still differ from an estimate.
The processing ceiling changes the choices
SaveFromNet limits a prepared job to 512 MB. An option known or strongly estimated to exceed that ceiling is omitted. If a size is unknown, a job can still stop when the real file reaches the limit. Select a lower available resolution for a long upload.
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Questions about this topic
Can a 25-minute 4K video really be tiny?
A very small number beside a long 4K video is suspect unless the source reports that complete file size. SaveFromNet now leaves the total unknown when it cannot account for video and audio.
Does 1080p always use less space than 4K?
Usually for the same encoding, but codec, bitrate, content, and audio can change the comparison. Check the options returned for your actual upload.