Master-Slave Database size difference

I have a Master-Slave replication setup running on Percona Server 5.6.32. Slave is read only and there are no other admin users with super privileges other than the one I hold. What I observed was the database size in Master is less than that of Slave. The database size in Slave is nearly 15-20G higher than that of master (I have queried on the database and also checked the physical data size on disk). How can that be possible if I don’t write anything on Slave? If the slave’s size is less than that of master, it atleast makes sense logically.

Can someone help me understand what is wrong here and how it works?