As Brogan hinted, the spaces allocated for the avatars there is about 4 times to the usual (2x width, 2x height).
Similar to this: http://xenforo.com/community/resources/avatars-big-tall.1741/ Essentially what happens is the CSS allocates additional space for the avatars, but to actually use a bigger avatar you need to edit the xen:avatar tag for that location to use the larger source image. It looks like you forgot that step.
