This is an interesting question. I just went out to look at two bikes I've got, both Santa Cruz, one of them a 26" Nomad from ten years ago, and the other a new Hightower 29er.

I'm pretty sure the derailleur clearance is actually lower on the old 26" bike! The geometry of the derailleur has changed, and when it's on the biggest rear sprocket, the old Nomad derailleur's center of mass is below, lower than the large sprocket. On the new 29er, the derailleur is mostly above the bottom of the largest sprocket. It seems like derailleurs have improved in order to allow the larger sprockets that single front chainrings call for.

The idler (lower pulley) on the 29" is almost certainly further from the axle than on the 26", however the larger wheel means that clearance from the ground is about identical.

I don't think I've ever hit my derailleur on either bike for lack of clearance -- even though I have caught my pedal on a rock or branch multiple times. I have hit the derailleur during a crash, but mainly because the bike was flying around and smacking into things. I'm guessing because the derailleur is behind and inside the path of a pedaling foot, it's mostly protected.