Producing small H.264 files with Adobe Media Encoder vs ffmpeg/libx264
Adobe licenses its H.264 encoder from Mainconcept, which doesn't do that well at low bitrates. x264 is pretty much the frontier when it comes at low size output for a given quality target, or quality for a given bitrate target. x264 is what's used by platforms like Youtube / Vimeo ..etc to encode user videos.

One thing you could try is to increase the keyframe interval. Default for x264 is 250 frames. Mainconcept will be something around 30-60 frames. If you can get them similar, I expect this to provide a moderate benefit in AME's encoding.

It is possible to use x264 with Adobe tools via VFW - https://sourceforge.net/projects/x264vfw/files/. But I don't recommend it. It's not maintained and there were stability issues.

My recommendation is to export a lighly compressed 1080p encode from AME - possibly DNxHD/HR codec in MOV or MXF container format, and then run the following ffmpeg command:

ffmpeg -i input.mov -c:v libx264 -crf 20 -movflags +faststart output.mp4


The faststart flag helps with quick playback start for web viewers.