Obsidian was very clever in this regard. They knew most people would simply compare the number of map markers between the two games and assume New Vegas having more must mean more locations. But that's not the case. They tricked you. Fallout 3 had dozens upon dozens, if not more than a hundred "minor" locations that were a good distance away from the nearest map marker, and were never marked themselves. Bethesda was honest, they didn't need to put a marker on everything. The majority of map markers marked a meaningful location. Obsidian, on the other hand, wasn't. I've explored 80% of the map so far and there are DOZENS of map markers that mark "minor" locations simply to pad it out. I can't tell you how many boarded up gas stations, houses with a single room, caves with nothing of interest, radiation-filled holes in the ground with a bunch of enemies to fight, and so on, I've found that are marked with map markers and would never have been marked in Fallout 3.
I don't know if it was a deliberate trick by Obsidian or if they truly thought every hole in the ground and gas station deserved a map marker, but I can tell you that comparing the number of map markers to Fallout 3 is extremely misleading.