in terms of quest length

map size

side stuff

building / cave layouts

Hmm, I would say it is longer in quest length, yes.

The map is much larger than FO3s.

FO3 had more sidestuff and easter eggs.

I would say Vegas wins in the building/cave area.

Difficult to answer. Many steps in the main quest are skippable. It's possible (but not desirable) to finish the main quest in a very short time. Then the game ends.
BUT -- If you play all the optional chapters of the main quest it will take a while.
Individually, quests run from very, very short to fairly involved; the game gets a lot of mileage out of you.



The map is irregular in shape which makes it look smaller but apparently if you count tiles it's a little larger. Don't be fooled by the paper map, which makes it appear there is a lot of unused space. It's just a rectangular frame.



You mean like side quests? Lots of them. Collectibles & loot? Lots of them, too, plus a crafting system.



In general, caves and buildings are small. There are a few caves that are larger than average and a few gigantic buildings. But, on average, smaller. Some are very well designed, though.

This game is geared toward story, rather than exploration. There's still plenty of the latter but it's not emphasized. With a few exceptions you can play this game without leaving towns or stepping off the highways.