This image is about ~330 tall pix by ~190 pix wide.
There is plenty of room in this to lengthen the avatar size to 150 by 150 without stretching the page width wise. The 2nd row of icons at the bottom of this (something which most users do not have access to) lengthens the size of this box by about 30 pixels. So, 50 pixels more vertical doesn't stretch the size of the page all that much.
Bu that should not really matter since people have the options of stretching their page well beyond this with longer posts or showing 40 posts per page rather than say 10 or 15. However, 150x150 increases the visibility of your avatar by a great amount for minuet details which gets annoying on 100x100.
Although, my guess is it takes more space on the site or the amount of work to get it to that point I suppose.