The Experience manager web service appears to be very similar to a standard Content Delivery web service, while its purpose is apparently just to transmit preview renderings of content to the Preview database. If that's all there is to it, it would require only a simple implementation to accept some data and throw it into a table or two.
Speaking of "a table or two" - the Preview database definitely identical to a normal broker database, as it's created in the same way. If you examine it, you'll find two tables that seem to have anything to do with preview content. So why use a full broker database?
Update: In @AlbertRomkes famous Troubleshooting XPM blog post, he mentions that data gets written to the COMPONENT_PRESENTATIONS table of the preview database.