Can't increase Mac OSX bash shell history length
You also need to set HISTSIZE=10000000.

HISTFILESIZE determines the number of lines to keep in the history file while HISTSIZE determines the number of lines to keep in memory.  Both default to 500

Execute echo $HISTFILESIZE $HISTSIZE before you make the change and then again after.  You should see 10000000 500 first then 10000000 10000000 after.