It's looks as though because it is an attachment, and it is a bigger picture, it automatically makes it a link. However if you are able to link it with the IMG tags I think it would simply resize it. I'll test it.
Yes, it appears the attached files that are too large function differently than similar images that are linked from other hosts.
Edit: Once I click on your attachments link it then turns the link into a smaller thumbnail in the thread, instead of just text.