Facebook paid $19 billion for WhatsApp since they were a huge success and a fundamental competitor to their business, whereas Disney had lots of options for making successful films and would have to put in a ton of work to get big money out of LucasFilm and thus paid $3.4 billion less than buying Pixar.
Value is based on the what the company is currently doing, their market position, the conditions of the market they're in, their trajectory, and how important the acquiring company feels they are to their business. It's generally always expensive, but you will see acquisition prices range wildly based on these types of valuations.
LucasFilm had a very valuable IP and wasn't doing a whole lot with it. King is a major leader in a market with huge barriers to success that's continuing to do well and has a large pipeline of products.
This is a very important point when comparing any major acquisition. I wanted to make it but Nirolak said it better.
I'm very curious how quickly Activision Blizzard will act. Disney made moves within mere moments of their acquisitions of Marvel and LucasArts. How much free reign will King have? It's a very interesting situation.