There were rumblings last month about one of iOS 9's new features, Wi-Fi Assist, eating into everyone's data plans. Now, if you're curious as to why that was happening, Apple's released a support doc describing exactly how the feature works.
Wi-Fi Assist essentially hops between Wi-Fi and cellular to find the best internet connection. As you'd expect, this can eat away at your data plan if you're not careful. Here's Apple's description alongside some caveats:
With Wi-Fi Assist, you can stay connected to the Internet even if you have a poor Wi-Fi connection. For example, if you're using Safari with a poor Wi-Fi connection and a webpage doesn't load, Wi-Fi Assist will activate and automatically switch to cellular so that the webpage continues to load. You can use Wi-Fi Assist with most apps like Safari, Apple Music, Mail, Maps, and more...
- Wi-Fi Assist will not automatically switch to cellular if you're data roaming.
- Wi-Fi Assist only works when you have apps running in the foreground and doesn't activate with background downloading of content.
- Wi-Fi Assist doesn't activate with some third-party apps that stream audio or video, or download attachments, like an email app, as they might use large amounts of data.
Which is all to say that, yes, Wi-Fi Assist can eat up your data, but it might not be doing it as much as you think. Either way, you can turn it off if you want to under Settings > Cellular.
About Wi-Fi Assist [Apple]