Different images have different compressions. With JPEG you sacrifice quality for smaller size and with PNG 24 you get alpha transparency. What you're probably more interested in is getting the smallest size image possible for the format you're using. There are 2 good command line tools for handling this. You kind of have to play around with different types, programs like photoshop make it really easy to see which is smallest. Once you've found your optimum format you can use a "smusher" to remove extraneous data that isn't needed by the image but often gets added by the editor. Below are 2 command line utilities I uses to make sure my images are as small s possible when building websites, but are also beneficial for any application.
optipng: http://optipng.sourceforge.net/jpegoptim: https://github.com/glennr/jpegoptim#readme
both can be installed easily using homebrew: Link and I'd assume theres an option for macports as well.
There also C-based so either can be used in an iOS project.