What determines the size of Illustrator file previews in Bridge?

What determines the size of the preview of an Illustrator file that you get in Bridge? Here is a screenshot:

The artboard for the file on the left is 4" x 4"; for the file on the right it is 1.7" x 1.7". So what is controlling the size of the preview, given that it obviously isn't the size of the artboard?