I was curious, so I looked into it. He's using the PNG file format. Since PNG is a lossless format, it's going to be larger by default. But the videos are the real reason it's so large. He encoded them with some sort of ultra high quality settings. I reencoded them using HandBreak using h.264 at 30 fps, and it took up a small fraction of the size with no noticeable loss in quality. The Bunnybf3.webm file went from 177,048 KB to 5,914 KB.