Confused about nautilus thumbnails

[Nyall <nyall zombiepigs net>]

I'm trying to create a thumbnailer for .gcs files, using the
instructions from
http://library.gnome.org/devel/integration-guide/stable/thumbnailer.html.en

I can get the thumbnailer working fine, and it all shows up nicely in
nautilus when the view size is 100%, but when I zoom in on the
thumbnails in nautilus they get horribly pixelated. This doesn't happen
with image files - I can zoom in and the thumbnails are recreated nicely
and look fantastic. So I've been trying to track down what's going
wrong. 

What I've found so far is that whenever nautilus calls my thumbnailer,
it's requesting a size of 128. This is regardless of the current zoom
level in nautilus. Digging a little deeper, I suspect this could be
happening with a lot of different thumbnailers - eg, the evince pdf
thumbnailer seems to have the same behaviour. 

So I'm confused - is this expected behaviour, a bug in my thumbnailer,
or a bug in nautilus?

Any help would be greatly appreciated!
Cheers,
Nyall