I don't buy the more than one monster theory (not counting the little parasites that fell off the big guy). The size seemed pretty consistent to me. (I had this discussion with one of my friends the other night) and I think the reason the monster seemed so tall in some scenes and not others is because (at least the way I remember it) the monster walked around hunched over most of the time, but had very long "arms" (almost as long as it's body itself) and in a few scenes (specifically where it attacked the helicopter) it reared up essentially making it appear 2-3 times taller. And it also had a pretty long tail, which is what I think hit the bridge. (I can't find any good pictures of it though, so I'm only going on what I can recall)rkolter wrote:(spoilers below)
There seemed clearly to me to be multiple monsters - the big momma in the city wreaking havoc with paws that reach twenty stories up buildings and span ten stories across, the smaller one in Central Park that would waste it's time eating one guy, the whatever it was tentacled thing that hit the bridge, the small spider-monsters... I could see several sequals with this array of monsters attacking cities.
And as for it going out of it's way to attack one tiny little guy, I though it odd too. I also thought it odd that went out of it's way to attacked the helicopter while it was being bombed. To me it seemed that it wasn't just acting like some wild animal, but that it actually wanted to kill people. Which I thought was cool and creepy.
Edit: Jim beat me to it.
