Check your supersize variable in the upload options of admin do you have it set to something like 250? This would be your issue.
There is no issue with the coding on your site that I am seeing. The large images are correctly being shown. It would appear to me you are intentionally setting supersize to a small size of 250pixels. There based on the supersize setting as it states larger images will be sized down. The software is simply doing what your telling it to do.
Maximium side for a supersized image? (Larger images will be resized down)
I use a supersize setting of 1024 personally and the max image height width in ads is 250pixels.