ibdata1 grows exponentially when innodb_file_per_table is configured
ibdata1 can still grow despite innodb_file_per_table being enabled. Why ?

Question: If InnoDB tables and associative indexes are written to individual tablespace files (file extension .ibd), what information still needs to be written to ibdata1 ?

Answer: Here are the following classes of information written to InnoDB's system tablespace:


Data Dictionary
Double Write Buffer


Safety Net to Prevent Data Corruption
Helps Bypass OS for Caching

Insert Buffer (Streamlines Changes to Secondary Indexes)
Rollback Segments
Undo Logs
Click Here to see a Pictorial Representation of ibdata1


I have discussed this before:


Dec 09, 2011 : What is the best way to reduce the size of ibdata in mysql?
Apr 01, 2012 : Is innodb_file_per_table advisable?
Mar 25, 2012 : Why does InnoDB store all databases in one file?


EPILOGUE

Given enough transactions, rollback segments and undo logs to support REPEATABLE READs can make ibdata1 grow. INSERTs and UPDATEs to InnoDB tables with Secondary Indexes can pile up  in the Insert Buffer. The Double Write Buffer Provides a second level of data redundancy in the event of a crash and subsequent crash recovery on mysqld startup. The only thing that can never grow is the Data Dictionary (unless you have DDL statements to create new InnoDB tables).