The YouTube favicon.ico is 318 bytes long. I threw together a crappy SVG mockup of the logo with absolutely no size optimization, and it clocks in at 429 bytes. When gzipped, they clock in at 233 and 248 bytes, respectively.
If you're worried more about the 15 bytes than about compatibility, then just have the logo scale up and use it for your main logo as well; it should actually save bandwidth that way.
If you're worried more about the 15 bytes than about compatibility, then just have the logo scale up and use it for your main logo as well; it should actually save bandwidth that way.