There are many reasons why a small dog may bite a large dog. These reasons may include:
A small dog may feel fearful or threatened, and biting an older dog may be a defensive reaction.
A small dog may feel dissatisfied or upset, and biting the older dog is one way of showing it.
Puppies may bite older dogs because they feel lonely or anxious.
A small dog may bite an older dog because it is irritated or bored.
A smaller dog may bite an older dog because it feels angry or provoked. The smaller dog may bite the older dog because he is dissatisfied with the older dog's behavior.
Puppies may bite larger dogs because they don’t have enough opportunities to socialize and play.
There are some other possible reasons, but in most cases, small dogs bite big dogs It's because they feel uneasy or have some unmet need. To help your puppy get rid of this behavior, you can try to provide it with adequate opportunities for socialization and play, and train it to learn to better cope with upset or anxious emotions.