The file sizes of the different thumbnail sizes. Baseline shows all the different sizes they used before, and current thumbnails shows the space saved by switching to just two thumbnail sizes.

They chose really poor colour gradients to show the differences.

The image would have been clearer if it used more than purple and what looks like the most minor variation on light blue.

The way I read it is that they eliminated intermediate thumbnail sizes. So the graph shows the before and after of storage space used by generated thumbnails.