Of course, they only had like a month for each DLC. I still like Honest Hearts the most. The landscape is superb, imo and always after visiting Zion, I find the Mojave to be extremely boring to look at. I even thought about creating a new landscape from the Zion set, because I want to have MOAR of it. Painted a sketch and all that stuff, but somehow never started working on it... Oh well.
The "one month" info is nice piece of bullshit. Release dates: Fallout: New Vegas - 19.10.2010 Dead Money - 11.02.2011 Honest Hearts - 17.05.2011 OWB - 19.07.2011 Lonesome Road - 20.09.2011 So, 4 months for first DLC, 3 for second, and 2 for last ones.
The dates you are presenting aren't development time, but when the DLCs were released. I'll guess they worked on them between 1 to 3 months at max. Still, the Zion worldmap has to be finished in a very short time, because you can't test and bugfix the game in a not existing worldspace. I think Sawyer wrote about the development time on Formspring long time ago. Someone might want to search for it- or simply ask him again.
Ever come back to a forum after a year or two and go to reply to a thread then suddenly realize, "hey wait a sec, I'm the OP" d'oh >.<