Well, it could be that muscle mass loss would reduce width. As for height, who knows, maybe for some reason the foot flattens (the bones would position themselves in a more flat pattern). For both of those, maybe skin and fat changes play a role too.
As for length... there's probably a better reason, but the only things I see now are that toe bones would grow farther apart, taking the room left by muscle, and making for a shorter foot because of the increased angle; also, I'm not sure why they would, but maybe the stuff in between each bone shrinks; finally, I noticed on a chart that there's a good centimetre in the big toe between the actual tip and the tip of the bone - maybe there's some room for explanation there.
And for patients in their 20s, there remains the seemingly improbable possibility of bones themselves shrinking, since they might not all have fused.
Really, it's hard to explain and it would take more of an expert than I, but with the number of reports, even if my hypotheses are entirely wrong, there just has to be some logical reason for it, and it's a true phenomenon I think we shouldn't deny. But seriously, considering how (especially) shaky my explanations are, I'd really like to know what the real reasons are.