The size of websites is commonly a metric related to performance and loading times. This tool puts a twist on that and uses the metric to instead highlight an other aspect:
Find out how much it costs for someone to use your site on mobile networks around the world.
Creating heavy, bloated websites is not inclusive. It excludes people on metered mobile connection that are paying for every megabyte they download (also people on slow network connections, with old devices etc.etc.).
The website of this tool also visualizes some calculations on how much loading an average website will cost a user in different countries around the world.
This topic was also discussed in the first a11yBerlin meetup back in January 2018