Version: 56
OS: Windows

Chrome .ico files currently store many image sizes, from 16x16 up to 256x256. All sizes <256 are stored as raw uncompressed BMPs while the 256x256 size is stored as a compressed PNG.

Support for PNG icons was added in Windows Vista, with the uncompressed icons remaining for compatibility with Windows XP. Now that we no longer support Windows XP, we can probably compress all the images to save space.

Example saving:
INFO: chrome/app/theme/chromium/win/chromium.ico : 46741 => 32287 (14454 bytes : 30 %)

Saving 14 KiB (times the number of icons) in the binary size ain't bad (though the actual download saving is likely much less since presumably it's compressed at some stage).

We could also probably remove the 8-bit versions (which are there for some kind of backwards compatibility also).