As I recall, a couple of years ago, APC stated they were going to stop making large one-piece props because of a few large props failed at the hub and they couldn't find the failure mode (they believed the design margin at the prop root was more than adequate and didn't understand why a prop would shuck a blade like that). If memory serves, they were talking larger than 18" or 20"...I don't recall exactly where they drew the line. All this stuff was on their web page some time ago...

However, I noticed that they have started making the big props again (perhaps because the hobby is moving toward ever larger planes and thus larger props). There isn't anything on the web site anymore, but it would appear that their answer was to beef up the prop root at the hub at the widest possible point. If you look at their latest large props from the front, the blades meet the hub nearly tangent to the hub diameter, providing much more material where they were failing before. I have to admit, they are pretty gruesome, but fortunately most of this extra girth is hidden in a spinner. I don't think the tips are all that different, but with such a massive hub it looks different...