Additionally the images are not very compressed (edit: you mentioned that in your comment, sorry). While the RAW files can be a couple dozen megabytes and the losslessly compressed PNGs are still 5-15 mb, good cameras normally set the JPEG quality factor to a high amount and so even with a JPEG you're getting pretty close to a lossless image. Whereas in video you can often plainly notice the compression artifacts & softness. A more fair comparison would be the video file to a JPEG with equivalent visual quality.