Now that I think about it, did Chris mean the Strip was bigger than 4 Megatons, or the city of Vegas itself? If it's just the city, that's a bit meh, IMO. Don't get me wrong, over 4 Megatons is still pretty big, but I was hoping something matching D.C...
I think he was saying that each of the four casinos on the Strip was equivalent to Megaton--I don't know if he means in size, number of NPCs, number of vendors, quests, etc. Taking everything else on the Strip into account (exterior environment and additional NPCs, other locations besides the casinos), the Strip is MUCH bigger than four Megatons.
And of course the Strip is just one part of Vegas--the rest of the city is broken up into various districts like Freeside that each have their own minor factions, quests, and in some cases, casinos. I expect that the city as a whole is likely comparable to the size of DC with all of its neighbourhoods.