If I remember correctly, 100x100 is actually an increase over what the site originally had. (I think we started at 75x75 when the site first opened)
Overall jasper is right. The larger the image size we allow, the more storage space we need. Ideally, we like to save as much storage space as we can for the database for the forum and for the Directory (the Directory hosts images as well). Perhaps in the future, if we make a move towards revamping the website, we can include some auto re-size feature for avatars in the upload process, but I think the general stance has always been that people here have a handle on at least simple photo editing, and therefore they can re-size and crop images appropriately for their avatar here.
Plus as a community, I think having an avatar separate from a YouTube channel is more personable since we a community based around people rather than a community based on a product like YouTube. Just my two cents on the idea though. Feedback surrounding the website is always appreciated, and that is why we have this Forum.