The burn feature in iPhoto is meant to be used as an iPhoto backup and not for simply burning photos to be used in anything but iPhoto. That is probably where the other files are coming in.
If all you want to do is to put some photos on disc, to give to someone else perhaps, use the export function to export the desired photos to a folder on your desktop and then burn that folder to a disc. Using the export function will allow you to select the size/resolution of the photos, and you will probably want to use the largest size for the best looking photos.