***Neither a teacher nor a native speaker.***
1. There are several big fat pandas - you're one of them.
I'm the big fat panda.
1. You're the only big fat panda.
2. Someone is looking for a specific big fat panda and thinks it is you.
I hope I understood your question correctly.
Anyway I'm sure there are more and better explanations, but I must go out now.
Student or Learner