What are the benefits of oversize lower headset bearings?

The shift to larger bearings may in part be marketing hype. The manufacturer can say "look how big ours is, that make it so much better". Another aspect may be as a result of warranty claims or law suits. If they see a lot of cracked head or steerer tubes a design change will be required to protect the brand image. The larger tubes are stronger. The larger tube requires a larger bearing. The majority of the load is on the lower bearing as it is supporting the riders weight. The same principal applies to the axles. Larger bearings can support more weight. Hence the proliferation of thru axles. Most riders don't realize there are only a few brands that manufacture there own frames. Most frames come from just a few factories. From the standardization aspect it makes sense that most of the headtubes be the same size. The final reason I can think of is the way and where people ride. Riders are going places and doing things that would have been unheard of just a few years ago. We are at the point that the limiting factor for most of us isn't the bike but the riders ability.

One noticeable difference with a tapered headset is that the lower bearing can be moved inside the frame, creating a cleaner look. With a standard 1-1/8 headset, road bikes could easily get lower bearing inside the headtube and I would think that mountain bike producers also thought this was a good idea and therefore had to come up with a solution to get larger bearings into the head tube and a tapered headset was really the only solution that made sense.

There is more load on the lower bearing because it bears all the weight and receives all vertical impacts. Making the lower part of the head tube bigger also allows for bigger down tubes.