Aceth is on the right path. The presented file size is the uncompressed size that the image will be once deployed. They are stored in compressed form, according to the TechNet article ‘Windows Deployment Services: Frequently Asked Questions’. Under the heading ‘What happened to the Single Instance Store’ it explains how WDS uses two files for each .wim image. Here is the relevant passage:
A Res.rwm file that contains all the file resources for all images in the image group. The data within Res.rwm is single-instanced and compressed; therefore, the Single Instance Store service is no longer needed. In fact, with Windows Deployment Services, the storage size on disk for images within an image group is greatly reduced, and it is more efficient than the storage mechanism used in RIS.
Additionally, at about the 36 minute mark in the TechNet Video ‘Deployment Day Session 3: Deployment Using WDS’, the host Rhonda explains how single-instance works.
Hope this helps