Why don't you use the video settings (that will make the quality much worse) in the program you've rendered the original video? Also you could use a more modern codec like H.265 or VP9, these have in average the double of the quality in the same file size (or the half file size, keeping the same quality) as H.264 or VP8. The only disadvantage is that rendering will cost much more time. Another thing is that not all especially older mobile devices will be able to play the file, but since dtube will render a H.264 480p version they'll be able to play that.