Not necessarily true. You could probably get by with a 64GB or 90GB SSD. I have a 120Gb Vertex 2 and with Windows 7 Ultimate x64, Office 2007 and 8 or 10 games including Skyrim I have 35 to 40GB free.
I would recommend a 120Gb or larger model if you do not want to be careful with what you have installed at any one time though. For instance I have FEAR and FEAR 2 on my Steam account and those games are 15GB each. So you can run out of space quick.