Why are LEGO sets put in such big boxes? I am wondering why LEGO puts sets in big packages. All bricks are put in small plastic bags and from my experience the packaging could be at least half of what it is.

There is so much told these days about ecology (especially in the EU, where LEGO has its seat), the smaller boxes would lead to better use of containers, so less CO2 during transportation and so on.

I understand that larger box "promises" something BIG inside, but could also lead to some disappointment.

When I was a kid and had my own sets, I'm sure they were relatively smaller.
Actually TLG are listening, and in fact are replacing boxes with smaller ones right now as we speak.




  These new boxes will have a FSC (Forest Stewardship Council) certification “which lowers the CO2 impact of its packaging by about 10%.” You will be able to start seeing the smaller boxes this year and by 2015, all of the products will be in smaller boxes. With this change, LEGO will save about 4000 tons of cardboard which is about 18% of their current consumption. These new boxes will come from sustainable forest so less trees will have to be taken down.
  
  [...]  LEGO is also involved in other plans to lower their carbon footprint. About 25% of the emissions are from manufacturing of the bricks themselves so they’ve made plans to mold the bricks more efficiently. The company also has invested in wind energy to improve their energy consumption. 


Emphasis mine. 

Now, as to why the boxes were huge in the first place, I can't tell. I heard all kinds of theories over the years, from "so the parts have room to move and don't get damaged" to "it looks more impressive on the shelves". But as this recent move demonstrates, all these reasons could either be worked around, or were negligible to begin with.