What are the benefits of oversize lower headset bearings? Long time ago, bicycles had threaded 1 inch headsets and fork steerer tubes were always made of steel. Then, approximately the same time, 1+1/8 inch headsets, threadless headsets and steerer tubes made of materials other than steel began to be widely used.
The 1 inch headset always was bit of marginal, especially if used with steerer tubes made of other materials than steel, so the move to 1+1/8 inch headsets is understandable. Tubes with larger diameter are stronger.
However, recently there has been a trend to make the steerer tube tapered. So it could be the case that the lower headset bearing is 1.5 inch whereas only the upper headset bearing is the standard 1+1/8 inch size. Then standard stems can be used but at the same time the lower headset bearing is much bigger.
What are the benefits of this strange setup where the lower headset bearing is larger and the steerer tube is tapered?
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.