ICO is a short extension (.ico) name for an icon. This is the icon format used for windows and other apps.
This is a uncompressed binary format similar or identical to BMP. It is often 32x32 pixels in size or perhaps 48x48. In simple icons usually 16 colors are used with 1 pixel equal to 4 bits or 2 pixels per byte. However, it can easily get expanded to 1 pixel per byte (256 colors) or even 1 byte per color making it 24 bits per pixel. These expansions in size resulting from the color map can make the icon much bigger than it needs to be.