The truth is always the best way to go, imho. I'd just tell them the truth.. "I love a big tree so I got a big one because I was sick and tired of hiding the little one every time someone knocked on the door".
If for some reason you can't be that straight with them (perhaps they can't handle the truth), here's a good excuse. Someone at work bought this tree but his wife really wanted a smaller one. So he dropped it off at your place and you took it in the house because you didn't want it to die at the front door. Then, lo and behold, you woke up the next morning and the tree was decorated. You have NO idea how that happened, it's just one of those freak things..