Not sure what book you are looking at but the truth is that BWV rooms are about the same size as BCV rooms. What you are seeing is a mistake that keeps popping up once in a while. The original plans that Disney submitted when building BWV show the rooms to be significantly larger than what was actually built. That size discrepancy then crept into the materials used to sell BWV and then it was corrected and remained corrected for about 6 years. Then in the early 2000s when
DVC redid its website, the old, incorrect room sizes were resurrected in the information provided on-line and in some promo materials. They then re-corrected the incorrect information again about four years later.
For a comparison of room sizes that is accurate and shows that BWV and BCV are close to the same go to the Disney Resource Center sticky in these threads:
http://www.disboards.com/showthread.php?t=2823943