Unless you're talking about the wii, this is not true. Even the Call of Duty's are more than that, and Halo required two discs.
Besides, there are plenty of multimillion selling games that are won't fit in 4 or 8GB, and we're talking 2 years out from today at the minimum. Multi-disc games for 360 are getting more and more common as time passes, and PS3 games are not going down on size either.
I said most games, it's pretty obvious that 16GB will be the upper limit at least initially for the biggest blockbuster games. Since we'll likely see better texture/sound/video compression on the actual media itself, the effective space of 8/16GB in todays console terms will be at least 9/18 or 10/20GB respectively on 8/16GB cartridges.