Custom Images - Creating Larger Versions

Hey just playing around with the add_image_size for my theme, it works a treat for large images, crops them nicely, however if someone uploads a tiny image, I'm trying to make the size increase or at least crop to the same pixel aspect ration.

Any idea how to make a image increase in size if it is already smaller than the specified parameters set when calling add_image_size function?

Here's my example: add_image_size('mobile-header', '940', '412', true );

Thanks