I can clarify a couple of points.
Stability
The size of a ship is another myth perpetrated by many passengers, but in fact, the ship's stability has little to do with size. It is affected by the ship's design and how the ship is loaded. When considering sea keeping abilities, again the ship's size has little bearing, as ship design and scantlings are the primary factors. Personally, I would much rather sail on the old smaller, steam powered liners (if they were still around), rather than any of the massive, modern mega ships. On SS Oriana & SS Canberra we could handle most seas at 20 to 25 kts. Your modern mega ships would have to reduce speed significantly, probably to around 10 kts.
Cost Comparison
After spending 40 yrs both working for and cruising with Princess, we now sail exclusively with a "Premium" cruise line. We completed World Cruises with Princess and a premium line. While the base fare of the premium line was double the Princess fare, the final per day cost was identical, with the standards on the premium line, vastly superior.