Yeah, depending on the image putting a bmp into a zip is actually significantly smaller than a png. Well, bmp has no alpha channel, but just as comment to the size of pngs. pngs even uses the zip algorithm in a way that is supposed to be optimized for images, but apparently it is not. E.g. the tiles here are a lot smaller as bmp in a zip: http://panzi.github.io/mandelbrot/

Ok, it put them into one single zip and can't remember if it was solid or not, so it might be the cross-file compression that makes the major difference here.