The obvious answer is "input file is different to output file" but that doesn't appear to be the whole story. I have a UHD file 2160p, I am sending to a 4k TV over the internet. Plex transcodes it to 720p. It is not a bandwidth setting, or a quality setting as I have...
For other DNLA-compatible devices, Plex reads just about any digital media file and then does transcoding on the fly, adjusting performance and audio/video quality for available network bandwidth. It's pretty easy to set up, too. The home version of Plex is free, and you can serve to ...