Actually, it's a matter of configuration.
There are three options the forum can do in case of an externally-linked-by-URL avatar (as opposed to uploaded one).
Check the dimensions and reject if it's too large, use Javascript to check the dimensions and resize proportionally on-page if it's too large (caveat: if your browser has disabled Javascript and someone links to a over_9000 x over_9000 avatar it will be shown in full size screwing up the whole layout), or blindly stretch/bend all linked images to specified size via HTML.
(Admin -> Attachments and Avatars -> Avatar Settings -> If the avatar is too large)
Currently, this last option is active, but I'll risk Dragyn's wrath and switch it to "check size and refuse" - after all, your is of proper dimesions, 100x49, and forum's limit is 100x100.