The disk usage size displayed for each build in the Build History is sometimes incorrect. Once the Builds Disk Usage is recorded it is updated. This is misleading for users if the do some config change and then perform a build and then look at the disk usage size (which then sometimes is incorrect).
I believe prior versions of the disk-usage plugin didn't display disk size right away.

One way to re-produce this is to create a Maven build job and switch the "inactivate artifact archiving" option. Disk size should be smaller when inactived, but the displayed figure doesn't not show this until after the record builds disk usage task is executed.