Information_Schema.Tables reports different result in MySQL 5.5 and MySQL 5.7
Short answer:  The numbers are close; don't worry.

Long answer:

There are a lot of explanations.


Well, I guess Barracuda is 15% smaller than Antelope.  (This may be the main reason for the difference.  Still, many other things can cause differences.)
Are they a different ROW_FORMAT.
SELECTs can cause locking that leads to extra fragmentation of inserts/updates/deletes.
Is innodb_file_per_table the same?
How was the Slave data cloned from the Master?
Have you done OPTIMIZE TABLE on either machine.  (There is virtually no reason to every do so.)
5.7 has a lot of code changes over 5.5, but I can't think of any other issues that might directly show up in those numbers.
It are not likely that there are any data differences.  Percona has a tool that will check.
Is there a "better way to monitor?" -- Monitor for what?


Also,


When an InnoDB block shrinks due to updating/deleting, there is some attempt to merge the block with an adjacent block.  It may be that 5.7 does a better job than 5.5, hence leading to slower growth of the table over time.  (This seemed to be more of an issue in index BTrees.)
The "Data_free" includes only a fraction of the reusable space.  It should not be trusted for much.