Can you intentionally set various bitrate (VBR) with a big difference, like from 16mbps max to 1mbps min? Constant bitrate can be your problem. Premiere had this on media export menu, I can’t remember where others have this. And, if you want to upload it to youtube, see what size it has on a private upload by downloading it - they reencode every video themselves to their uniform standard, so maybe you don’t even need to bother with that.