Source loads precisely zero junk meaning that you can build your pages without a ton of needless bloat.
A RapidWeaver page for each of the tested frameworks was added to a new project. On each page the appropriate theme for the framework was added along with the 'base' stack. With only these things added the project was published locally.
The figures in the chart relate to the size (in Kilobytes) of various items that are loaded onto the blank page. These are non-gzipped file sizes. If a compression technology is used to serve the files then these sizes will be lower but this is not the default behaviour for most web hosts.
Source, of course, isn't a full framework (we have termed it a 'micro' framework) and it doesn't offer many of the options that are included within the other frameworks (galleries, accordions, sliders etc). It is therefore an unfair comparison in some ways. What this chart is intending to show though is simply the starting point for any project. Obviously, if you go on to use mostly native stacks in the various frameworks then you will be getting some of the worth of these file sizes.
Source is simply intended to give you a true blank page upon which you can build the sites you want using whatever stacks you choose to from the RapidWeaver Community (and of course the really great ones included with Source). Check out our Source projects to see that a 'micro' framework is no hindrance to creating great websites.