I use a piece of software called Free Ico Converter 1.3. It takes jpeg or png images and converts them to .ico. I sometimes have to resize the images in PS first. This is not the first time this has happened. It also happened on my laptop. Just, all of a sudden, out of the blue, some (but not all) of the icons just shrink. I have yet to figure out a commonality between them or a parallel conflict.
I read another place where rebuilding Windows' icon db is supposed to help. I got a batch file that's supposed to do it. I will try it when I get home.