The private cloud's market size is about $7 billion, according to the analysts at Wikibon. That's a quarter the size of the public cloud market, which Wikibon estimates as about $25 billion. Indeed, the value of Amazon Web Services (AWS) alone is larger than the entire private cloud market -- AWS's market share came in at $7.9 billion in 2015.
Other surveys show that the private cloud is still growing, but it won’t overtake public clouds. Indeed, I suspect that private clouds have peaked, and their growth will slow as enterprises increasingly focus on public clouds.
All of this shows the sea change in enterprise cloud thinking: Only a few years ago, enterprises proclaimed private cloud strategies so that they could own their IT assets, and they viewed public clouds suspiciously.
Why the change? One reason is that public clouds are now more capable than private clouds. Look what’s in AWS versus an OpenStack distribution -- it’s night and day. Ditto when comparing other public clouds to other private clouds.
But the biggest reason is that "private cloud" is another way to say "my own data center," which does not provide the benefits that companies seek from the principles of cloud computing. Both business and IT are increasingly acknowledging that "private cloud" isn't the cloud.
The "private cloud" notion has also been perpetuated by traditional enterprise vendors, who needed to cloud-wash their existing wares to be able to keep selling them. Oracle, VMware, HP Enterprise, and most major providers have been guilty of this. But that cloud-washing has lost its resonance as IT organizations have learned the truth in their own deployments.
Still, private clouds have a place. There remain legitimate reasons to own and manage your own technology infrastructure, and private clouds bring some of the benefits of cloud principles to such data center deployments.
At the end of the day, however, the private cloud is for your exceptions, not the norm. That is why enterprises will find more value with public clouds and continue to grow their public cloud usage accordingly.