Quicktime player: merging multiple videos results in much greater file size than original clips

I used Quicktime player to merge multiple videos. ### Info: - The file size of all the original clips that I merged is 3.06GB - Used HEVC codec - Source is also HEVC ### Resolution and size of merged videos - resolution of original videos: 1080p and 720p - 1080p filesize: 11GB - 720p filesize: 9.09GB - 480p filesize: 3.2GB Don't want to use 480p as there is a noticeable difference in quality (of course, but I tested to make sure). ### Question: Why is the file size of the merged videos greater than the original clips and is there anyway around this? ### More info: A huge increase of file size only happens when I convert to 1080p whether the source is 720p or 1080p. When converting to 720p whether the source is 1080p or 720p there is only a slight increase in file size.