Part of the space savings in these cameras comes from the removal of the mirror and the mechanical mechanisms that actuate that mirror. The iconic pentaprism on the top of an SLR can disappear and the body can get thinner and lighter because they eliminate the need to make room for a piece of glass that has to pivot up and down to capture an image.
The result is cameras and lenses that are much, much smaller and lighter than traditional SLRs. Remember, some of the sluggish performance of contrast-detection focusing is due to the weight of SLR lenses. Lighter lenses mean less mass to rack back and forth to measure focus.
But without that heavy, cumbersome mirror, the ability to divert light to a focusing sensor went away (at least at first; more on that in a moment), and as a result, Micro Four Thirds and similar mirror-free systems relied entirely on contrast-detection systems.
"The motors are getting smaller, faster and lighter," explains Nikon's Heiner of both the company's SLR and mirrorless cameras. "In DSLR lenses, we have the luxury of much more space. In mirrorless, the space is smaller, but they don't have to move as much mass, so [contrast detection] tends to work extremely well in that system. Where the Silent Wave motors used [in Nikon pro lenses] work very fast, they're power-hungry."
"The whole size battle...people have to understand that bigger isn't necessarily better," says Richard Sasserath, technical specialist at Olympus. "When you talk about an SLR, 90% of the time, they're using phase detect. With Micro Four Thirds with contrast detect, you're getting a much faster autofocus."
In fact, Olympus has clocked some of their Micro Four Thirds cameras as having the fastest autofocus in the world—besting phase-detect-based cameras.
That's because manufacturers, having eliminated their ability to include phase-detection systems in the mirrorless cameras, have had to focus a lot of time and energy on making contrast-detection focusing faster, and more accurate, to boot.
"It's one thing to say it's the world's fastest," Sasserath adds, "but it's also incredibly accurate. You can be fast and not focus on the right spot, and what good is it? We have to train people to realize that phase detection doesn't mean the fastest in every setting."
It doesn't matter if you don't know where the subject will be in a few seconds, as long as you can focus so fast when the time comes that you can capture the subject in focus before it has moved out of focus again.
So the rules have changed, and now it's no longer true that phase-detection systems perform best in all conditions, and in fact, contrast detection can be a bit faster in some applications.