Image not being compressed/Image size too large

[panosl]

Trying to make a thumbnail out of this image:

http://www.menufesto.gr/media/attachments/marketplace_store/86/kei.jpg

which is 644KB (978x978px).

What sorl outputs is this:

http://www.menufesto.gr/media/cache/52/98/52989b5eeb5c08d0cebe13e6449f4118.jpg

which is ~560KB (150x92px).

Here's the source that handles it:

{% thumbnail store_attachments.0.attachment_file "150x92" crop="center" as thumb %}
    <img src="{{ thumb.url }}" width="150" height="92" />
{% empty %}
    <img class="empty" src="/static/img/empty.png" />
    <span class="button_text">{{ store.name|title }}</span>
{% endthumbnail %}

and the settings.py:

        THUMBNAIL_DEBUG = False
        THUMBNAIL_QUALITY = 85

I tried cropping and resizing it manually with GIMP and the resulting image is 7KB. What's going on here?

(GIMP output)