So there is a certain amount of ebb and flow you're kind of overlooking here.
Taking a base constitutes an investment, and not a light one either. We're talking about 350+75*2+50*22(or possibly 30). Even on 2-3 Bases this constitutes a heavy investment that could have gone to troops. If you hit mid game and your opponent is suddenly on 5 bases and you're on 2, that means his army is going to be about 4800 minerals smaller than yours (approximately 75-90 supply for a Zerg). At that point it doesn't really matter what he has your army size should sort of crush him.
So given that an expansion is such a heavy investment its not hard to see why many players build an army before taking a 3rd or 4th base. Usually even early expansions are vulnerable to pressure early on.
Every time your opponent takes a base you should consider it an opportunity.
If they are taking 3 more bases than you have, that is 3 times the opportunity.
However, if you miss an opportunity like that, and they do get those 3 bases up and running their economy is going to start kicking in and the are going to bowl you over. This is why so many top players go for a Macro oriented game, because its the closest thing to a
sure thing Starcraft really has.