Funny. On my iPhone it is the complete opposite.
I cannot see anything in thumbnails unless they are third party hosted because the full size image is always very small
AND it resizes any large images automatically to fit the screen so I can see them great or click for a really big image.
I am afraid I am guilty of the big images often as that is Flickr default.
I wonder if there is a way to set your preferences to display images at a particular size?