Difference between RTRIM and switching from VARCHAR to CHAR and back to VARCHAR

[Eduardo Koerich Davila]

What I'm going to post here is something very curious to me. I didn't find an answer yet and was hopping if someone has already faced the same problem.

I was importing a TXT file into my database using fixed length columns, that is, without any separator. At some point I realized that many rows had trailing spaces in many columns and I was wasting space on my storage. So, I thougth, let's change the datatype to VARCHAR and, after importing, UPDATE every row using RTRIM to remove the trailing spaces.

OK, everything went just as expected. I reduced the size of my database, but the diference wasn't that high. For some reason, don't ask me why, I changed the columns back to CHAR and than to VARCHAR again. For my surprise, after that, the database was with half of it's size, without affecting the data inside. I did the same procedure with MyISAM, but the awesome result just ocurred when using InnoDB.

Does anybody know why is that and how can I control this behavior?