Wow, glad I found this post, I was having the same problem with a ~6GB database here. Trans logs were growing to 6GB unexplainably during the night, and we determined the maintenance plan was definitely it!
I've been using the Maintenance Plan wizard to setup daily "Reorganize data & index pages" along with daily full backups for years, on databases ranging from a few MB to 400+GB. I've never, ever seen the transaction log grow like that because of a maintenance plan before. I've always used the Full recovery model on all my databases.
Does anyone know what situation specifically causes the optimizations job to make the trans log grow so much? Bad disk fragmentation? Lots of transactions during the day?