Steam games actual size

So living in New Zealand, we have the usual smaller download caps per month, which means I have to really pay attention to how big Steam games are before downloading. 
 
Although sometimes when I do download games, they are not exactly the same size that the 'recommended HDD space' tells me. 
 
My question is, is there a rule of thumb as to how big each game actually is to download? 
 
Say a games recommended HDD space requirement is 10GB, does that mean the Steam download is 10GB?

Yeah the download is pretty much that size. Some may be a bit smaller due to compression in the installer. The source engine based games won't be as big after you have at least one source game installed due to shared resources.

Well before you install a game onto Steam, it shows specific hard drive space requirements. But that's only for bought games though unfortunately.

They are what they say they are from my experience. 

Is there any real reason they don't compress the games a bit more? It would help a lot of people.

The HDD requirements aren't exact - for example, S.T.A.L.K.E.R. Call of Pripyat recommends 6 GB, but it only actually takes up 5.24 GB. Serious Sam HD recommends 1 GB, but it actually takes up 1.05 GB. Still, it's close enough that it shouldn't really matter.

" Is there any real reason they don't compress the games a bit more? It would help a lot of people. "