It might help if you pointed out the ones that are larger for reference. As far as I can see, there are two ways that your picture could be made bigger.
Widen it - At the moment it is only 67 pixels wide. You are allowed an avatar up to 80 x 80. It is already 80 tall so you can only widen it based on the size criteria.
Simplify it - The criteria is 80 x 80 or 19.5kb
whichever is smaller. So if you have a simpler avatar to meet the file size restriction, it can theoretically be larger. I've tested this with an extremely basic 100 x 100 avatar, but that appears to have been automatically restricted to 80 x 80 on the forum. Can you point out any larger ones showing up?
Nice signature quote you have there.
