This is normal for the following reasons.
One. It's related to your own genes. The chest is one big and one small, but it is not obvious because the pectoral muscles are underdeveloped.
Two. Irregular bench press action leads to long-term deformation of chest muscles.
Three. When bench press, the attention is not completely focused on the two pectoral muscles, but on the other side.
Continue to practice like this, pay attention to the correct posture and pay attention to the insufficient side. When the developed side is big enough, it will not be big again, and then the other side will grow up slowly.
Hehe, I hope I can help you.