Although it takes a bit longer to warm up (a bad thing), I'd think that less sump capacity is NOT a good thing, simply because the oil will contaminate faster. As for ever-smaller sump capacities, I suspect the following:
1. As we'd all figure, smaller engines require smaller sumps -- no surprise there.
2. Smaller cars mean smaller engine compartments, meaning smaller sumps and smaller everything.
3. Newer engines tend to contaminate their oils less due to more sophisticated sealing, somewhat tighter tolerances, and longer overall engine life.
4. Newer oil base stocks and additive packages are better able to cope with contamination, therefore requiring less oil to accomplish same.
5. Typical oil filters have gone from 35-40 micron filtration of two decades past to about 25 micron today, better keeping particulates out of oil.
6. As mentioned before, we're seeing good UOAs and engines lasting longer in spite of this, so none of this is detrimental in the big picture.