On August 14 1945, the Navy had in active service 23 battleships, 28 fleet carriers, 71 escort carriers, 72 cruisers, 377 destroyers, 361 frigates, and 232 subs: a total of 6786 ships, including auxiliaries. The total personnel strength was 3.4 million.
Imagine that, we had a shitload of active warships and manpower in the US Navy fourteen days prior to the surrender of Japan during WWII. That was almost 4 years after Pearl Harbor. What was the US Navy looking like in 1939? Nowhere near what it was at the end of the Pacific campaign.
On September 30 2006, the Navy had 0 battleships, 12 carriers, 27 cruisers, 54 destroyers, 35 frigates, and 74 subs: a total of 318 ships including auxiliaries. The total personnel strength was 0.35 million.
And what was the destructive capacity of the Navy in 2006 compared to August 1945? Hell, one Ohio class submarine has more destructive capacity than the entire Navy from 1945. As cool as battleships are, they are a relic and have no real function in the current military. A single carrier group from the current Nimitz class could obliterate all 28 fleet carriers and support ships before they even knew what had happened. Technology has made the requirement for massive amounts of ships meaningless. The amount of manpower is also significantly reduced. You also can't compare the necessary number of ships during a massive multi-year war to post cold war times. Iraq, Afghanistan, etc are regional conflicts at best and not even against the country itself. Massive amounts of firepower are generally not wise when fighting insurgents. You don't carpet bomb an entire village when there are only 4 hostiles in it. .
The size has shrunk considerably since 2006. This despite having twice the national population to draw upon. The Navy can barely man its ships, let alone sparing "manpower" for non-essential tasks. And just like the merchant marine, knowledge, specialties, and capabilities have been cut way back. There are no more hundreds of sailors manning the engine rooms in large ships. The engines are automated.
The cold war is over and the US is not at war with any large governments any longer. Why would you want more men when the ships have become more efficient and have so much more firepower? Look at the number of men in the Iraqi military compared to the US. How did those superior numbers work out for them? In the case of carriers they are also nuclear. You don't need men to shovel coal into boilers any longer either.