I made a backup to proceed with the flash, but I noticed it has a very small size (the phone says it is about 150 megabytes, on my windows file manager my /.backups folder contains files for about 80 megabytes).

What does it really backup? Are SMS messages in there?

I also copied the whole content of MyDocs folder as a full backup, so I'm sure at least photos, wallpapers and music are there.