wans't this the spell arthur used in the ford anglia? in the Chamber of Secrets there's an excerpt explaining this: HouseElf13
"Harry couldn’t see how eight people, six large trunks, two owls and a rat were going to fit into one small Ford Anglia. He had reckoned, of course, without the special features which Mr Weasley had added.
‘Not a word to Molly,’ he whispered to Harry as he opened the boot and showed him how it had been magically expanded so that the trunks fitted easily." --