Beat me to it while I was editing.
It's a relatively low pressure round, typically running below 20,000 psi. That's too low for the brass to stick in the chamber, so the whole 45 Auto case backs up against the breech under firing pressure, where the taper of the chamber makes it wider, then expands to fit the wider space, shortening the case. Upon resizing, a tiny bit of the brass flows back, so the case can't 100% recover the length it had before firing. In other words, as Nhyrum said, they actually shorten with use.
I once tracked on a bulk lot of Winchester brass I bought new through 50 reloadings of light match loads. About 2/3 had been lost or split by the end, but every one of the remaining cases was about 0.025" shorter than it had been originally. So they were losing an average of about half a thousandth at each reloading cycle. That is too short to headspace on the case mouth (the rim is stopped by the extractor on a 1911 before the case mouth can reach the end of a chamber when the brass is that short), so I loaded the bullets out to stop on the throat as the headspace determinate (see image below, third image from left). This proved to improve accuracy and reduce leading, and the cast bullets were soft enough and the loads light enough that it didn't raise any pressure issues. I did have to adjust my taper crimp die down as the cases grew shorter, though.
