Rollup's gains come mainly from the way it combines modules into a single scope, rather than wrapping them in functions and shipping a module loader with the bundle. It's not uncommon to see parse/evaluate time plummet when switching to Rollup because of this, which is often more important than the number of bytes.

However Rollup currently lacks a lot of features that make Webpack a good choice for large and complex apps.

You can get a feel for the code Rollup generates at http://rollupjs.org.