So, this doesn't contribute directly to OPs question, but I bought my mom a new VW Taos recently that has a flat-bottomed steering wheel, the first I've ever driven with. I have to say, I really like the way it feels when turning. Fat/big people aren't necessarily the only ones benefiting from flat-bottomed wheels, seating position can make a difference to the utility derived from that type of wheel design as well: I drive all my cars with a higher seating position for a better view, but my steering positioning is always the lowest possible since that's what is most comfortable for my arms (I'm a fairly average physical specimen at 5ft 11", 215 lbs) and I immediately noticed that the steering wheel just felt more comfortable. While turning, I got used to grabbing the flat part with my right hand and found it easier/better for one-hand driving.
