Battery life will vary depending on the types of applications you are using. iPad Pro is a supercomputer capable of running some of the most powerful and advanced applications in the world. When you are running sophisticated apps, they will reduce the runtime your iPad battery can manage.
Yes, the Pros are definitely much more powerful. That said, I'm still using the same apps on the Pro that I did on my older iPads: comic reader, ebook reader, Safari, Mail, Netflix. With similar usage and brightness settings, the iPad 4 lasted longer than my Pro 9.7.
Of course, the iPad 4 had a 43Wh battery. The Pro 9.7 only has 28Wh and even the Pro 12.9 only has 41Wh. Apple sacrificed battery for thinness and weight. It makes a lot of sense for the iPad given they had a lot of leeway there.
With heavy usage, I think I only charged my iPad 4 once every 3 days or so whereas I need to charge the Pro daily. Still, the Pro does last me an entire day of usage (~10 hours onscreen time) whereas even the iPhone 6 Plus couldn't deliver the same level of onscreen time. I'm more than willing to sacrifice a few mm to get bigger batteries on the 4.7" iPhones.