Like any other type of software, also Web Application Programming Interfaces (APIs) evolve over time. In the case of widely used API, introducing changes is never a trivial task, because of the risk of breaking thousands of clients relying on the API. In this paper we conduct an empirical study over a large collection of OpenAPI descriptions obtained by mining open source repositories. We measure the speed at which Web APIs change and how changes affect their size, simply defined as the number of operations. The dataset of API descriptions was collected over a period of one year and includes APIs with histories spanning across up to 7 years of commits. The main finding is that APIs tend to grow, although some do reduce their size, as shown in the case study examples included in the appendix.