WebM and MP4 are containers, not video codecs. You can't assume anything about the efficiency of the video just by knowing the container. In this case, the WebM

WebM and MP4 are containers, not video codecs. You can't assume anything about the efficiency of the video just by knowing the container. In this case, the WebM files contain video encoded with the VP8 codec, which in my opinion is significantly less transparent at the same bitrate than the h264 codec that the MP4 file contains. But it's more likely that whoever encoded these files did so without trying to hit specific transparency targets, so you shouldn't really conclude anything from the file size.