l was watching a video on funnyordie.com and Will Arnett stated that he liked his Hummer because it was big enough to perform an abortion in it.

Here is the video:

http://funnyordie.com/videos/4f551b0252/will-arnett-human-giant-sex-tape-from-human-giant-and-will-arnett

Are Hummers really that big inside? l actually heard that they were just bulky and were not very big inside at all.

It would depend on how the insides were loaded. If it is the striped out model with only the front driver and passenger seats then yes it is very possible. On the other hand if it has both the front and rear seats then it would be very cramped and l would have to question it.
Some of those military hummers come with only one bucket style drivers seat leaving a lot of room. A good doctor could probably do double abortions, his wife and his girl friend.