Why is Zerg considered behind if they don't have 1 more base than their opponent?

Often times in gamecasts, I hear the commentator saying how the Zerg player must always stay one expansion ahead of the other player, most importantly in the start of the game. Why is that? Wouldn't you consider that imbalanced? Or does it balance out somehow by Zerg being able to expand with more ease?

Solution 1:

Zerg expansions are the cheapest to build (costing only 350 minerals compared to 400) in addition they can grow their economy the fastest of any races. To compensate for their extreme economic power Zerg units are relatively fragile (largely due to their lack of range), not particularly cost effective, and the Zerg player must constantly choose between drones and army. In addition, one of the more technological (protoss or terran) races can support more overall production (workers + army) without being handicapped by the larvae mechanic. The combination of these force the Zerg to capitalize on its powerful economy to expand early (providing more larvae to match its opponents production) with the eventual goal of crushing the more technological races by sheer force.

Summary:

Zerg's 1 base cannot support as much production as 1 base terran or protoss due to having to choose between workers and army (larvae). Combined with cheap expansions and a strong economy it puts them into a balanced position in the match ups with a macro oriented play style

Solution 2:

The main reason as Aardvark told is increasing of larva production. But often, instead of expansion Zerg build 2nd Hatchery on their main (for example, in 2v2 game been under opponent's pressure).