Windows 7 HP with SP1 is actually about 30GB. When you add all the updates it goes over 30GB. The 20GB recommendation was made back before 2009 when it was being developed and they have rolled out a ton of updates since. I also do believe that 20GB was for Basic/Starter editions but I could be wrong.
20GB was for base Windows 7 install - I think it was for all versions IIRC.
But yes, exactly like I was saying. 32GB wouldn't be enough after all the updates and other extra stuff that needs to go in for just the OS.