Can a ferret's tail become fluffy and large?

Yes. It is commonly called "bottle brush tail" as it resembles a bottle brush. When a ferret has a puffed up tail he/she is either very excited or scared.

If the tail is puffed up and the ferret is backing away /hissing it is scared and should be left alone.

If the tail is puffed up and the ferret is dooking he/she is happy