Coincidence.
Almost any ALTER rebuilds the table. In rebuilding a table, free space, blocking inefficiencies, etc are removed. That is probably the real cause for the data and sizes to decline.
InnoDB estimates the number of rows in a table (I am surprised to see identical values there.) Then, it derives the average row length from that and the data size.
(The discussion of how InnoDB stores data into BTrees, deletes rows, etc, is long winded, and covered a few times in the InnoDB forum.)
Now, on to the important question, did the conversion work? Suggest you do
SELECT HEX(colname) ...
and see if any accented letters now take 2 bytes.
Be sure to change the client(s) if needed. (SET NAMES, etc)
More discussion:
http://mysql.rjweb.org/doc.php/charcoll