When building world and kernel on DragonFly, /usr/obj is where the work files get placed.  This can eat a bit of space, but it can be safely deleted.  If you keep the files around, subsequent rebuilds can be done faster with a quickwork/quickkernel, but this may not matter to you.

(This was answered on the mailing lists by Max Herrgaard, but I don’t have a link to his reply – sorry!)