I went and took a look at your blog. At the bottom of the page, you have this:
Create a new virtual machine with the following configuration
3072 MB ram
100 Gig VHD on the D: drive
Your screenshot shows the Virtual HD 100GB as the primary drive. As I said, the "Total Size" reported by the Data drive is the size of the primary drive minus 20GB for the SYS partition, which equals the 80GB you are seeing.
This has nothing to do with WHS running on Hyper-V. This same behavior occurs on physical hardware. You could consider this a "bug" but again, it isn't something Microsoft is really concerened about as it does not affect the way WHS functions and does not even appear if the product is used as intended. Power Pack 2 is suppose to resolve any isses with drive mappings showing the wrong size but some people have reported they are still seeing the wrong size.
JJ