InnoDB table size increased substantially after ALTER
It's difficult to answer without knowing the nature of the data being copied and maybe of the integration software that is used.

But I decided to share my guesses hoping it may lead to better diagnostic:


Row length increase may be because texts were converted into Unicode during migration.
Lower row count maybe due to some kind of deduplication that also removed records with less text data thus increasing average row length. Or it might be just that the migration process was aborted in the middle.


It might help to make a more educated guess if you supplement the question with SHOW CREATE TABLE output for both source and target tables.