The high resolution would be used to size down to any size smaler than it, to not lose detail.
The alternative to that would be to size up fro a smaller image, which might not look so good, depending on how much you scale up.
The smilies/emotes are scaled to 112x112 or 128x128. Simply for the sake of: fitting in the tool panel, being a smaller file size (quicker loading), and, not taking too much space when used in the threads.