- The actual difference (minified and gzipped) is 17.6 KB vs. 5.7KB.

- Lodash is written modularly so you can import only the methods you actually need.

- In order to "lose some weight" (as Jeremy Ashkenas called it), underscore dropped several methods that people really wanted (including other contributors).

- Also to cut down the size, several major pieces of functionality is not broken in older versions of IE

- Lodash provides a lot more functionality than underscore does, there's a lot of additional utilities including the ones that were dropped last minute by underscore.

There are a lot of other reasons to use lodash, but that was quickly becoming not a _couple_ of things.