Difference in MYSQL Master Slave DISK SPACE
The tables on the Primary and the Replica are maintained separately.  All InnoDB tables are B+Trees.

A block might need splitting on one but not the other.
A long-running SELECT on one might lock rows in such a way that there are more block splits.
Small blocks may get recombined, but not necessarily with the same frequency, etc, on both machine.
Were all the same ALTERs done on both machines?
Which columns of SHOW TABLE STATUS show the most difference?

If it is a few percent difference, the above things might be the reason.
If it is 50%, something strange is going on; we need more details.  For starters, which one is larger.