I'm done with converting H.265 videos to H.264. I recorded a 2:30 min. 4K video last evening which was 1.7 GB in size. However, after converting the video to H.264 1080P using RockyMountain Movie Converter, the size ballooned up to 8.2 GB! My plan was to edit the converted H.264 video in Adobe Premiere Elements 12...but 8.2 GB...seriously?! PowerDirector 13 to the rescue! I imported the same original 4K video into PD13, edited the video to my liking and saved to H.264 1080. What was the size of the rendered 1080P video? Only 375MB! OK, I'm now off the soapbox.
I am the author of RockyMountain Movie Converter.
1.7GB 4K is converted to H.264 FHD in RockyMountain Movie converter and it ended up 8.2 GB? What did you set as "Quality" option?
If I remember right, RM converter converts NX1 4K to H.264 FHD about 1 to 2 times of size from its original size when default quality option, "10".
In according to FFMPEG manual, Quality "20" of H.264 encode means human eyes cannot reconize difference between original and encoded one. I set the quality "10" for H.264 as default and it is already very, very high quality.
I guess you might set RM converter quality option better than "10". It is very close to lossless option. For sure, the size will be humongous.
If the output size does matter to you, you can control the output size with "Quality" option in RockyMountain Movie converter. Why don't you set quality as "20"? It is already very good quality. Want to smaller output size? Try worst quality option "51". It willl end up with just few MB of out put size.
I haven't used PowerDirector 13, so I have no idea what was the conversion quality and speed option.
If you want to have good quality output? then the output file size will be bigger. If you want smaller output file size? then sacrifice the quality. There is no magic as long as you stay in the same codec.