66% of my customers on eligible devices have at least 1.3GB of free space [enough to install iOS 9]. This compares to just 37% of users who would have immediately had sufficient space at the old iOS 8 requirement.
via David Smith
Speaking as an owner of a 16GB iPhone, this is great news! I had so many of my friends tell me that they couldn't install iOS 8 last fall because they didn't have space. Most of them eventually upgraded by deleting apps, but some did not. This is a terrible user experience, and I'm glad Apple is addressing it in iOS 9.
With the news that iOS 9 will also automatically delete apps and reinstall them for you as a part of the install (if you don't have enough space), it seems that pretty much everyone will be able to upgrade.
But Apple, come on, be a pal and drop 32GB into the low-end iPhone 6S in September.