He Ain't Heavy. He's a Hobbit.

[Nufaciel]

For the scenes where the "Hobbits" had to interact with the larger characters (ie. Sean Bean picking up Elijah) They used stand-ins who were the proper size. For example, when you saw Boromir's facing the screen and you couldn't see Frodo, there was a stand-in that was the size of Frodo. When Boromir was holding Frodo, but you could only see Frodo, it was a 7-foot something stand in. They had two different sizes of sets so that they could film each part in the appropriate height, and then spliced them together. I've seen this all somewhere, scattered throughout various books on the movie, magazines, and websites. You might want to try the archives on theonering.net. Maybe someone else can explain better. I'm still reeling from the hot scene I saw of Elrond on the 10 minute TTT preview on the DVD.