When you paste a bare link to a Stack Exchange site in chat, it gets oneboxed as a clickable image of the site's logo, like so:
When the site's logo is enormous, like CS Theory's, it overflows if the width of the chat area is less than the width of the logo. This causes the image to hang over into the side bar area, and go off the page if the window is small enough:
This of course doesn't look so good. Typically images that are one-boxed get scaled down to fit nicely in the chat window; the same size restrictions should be applied to one-boxed site logos as well.