I, too, was amazed how much you could stuff into the box! I managed to get in the bed, a pony, and many, many little "outfits". There are tons of Disney princess outfits, plus Minnie Mouse. Too cute!
There are some tricks to maximizing the space - put some soft clothing-type items in the bottom of the box, then put the bed in upside down. Stuff some more soft clothing items around the edges of the bed.
It's quite fun to do this! All of us adults were on a zealous quest to get all of the pieces!!! Because most of the items are so small, it's easy to miss some of the pieces for each pony "outfit" (particularly things like the shoes, which are heavier and sink to the bottom of the bin, and the tiny items like necklaces and sunglasses). Have one of the CMs help you to make sure you don't miss any of the pieces. (I got a "dud" for a CM - she didn't help me at all, but one of the other guests helped me figure out which pieces I was missing.) I spent about a half hour there before I had my box stuffed. My niece will be getting her pony "kit" for Christmas - I can't wait, she's a MLP fanatic!
Have fun!