>But it seems dangerous to me to start with a native format with this behavior and attempt to clean it up later. File formats don't have behaviors. Programs do

>But it seems dangerous to me to start with a native format with this behavior and attempt to clean it up later. File formats don't have behaviors. Programs do. Lots of binary formats, including PNG, will ignore any extra junk after the end of the file. So if a change makes a PNG file smaller, there's nothing stopping a program from leaving data from the old version at the end. But that's not normal behavior for programs that write PNG, so it's not an issue. As long as programs adopted the behavior of VACUUMing the database, this wouldn't be an issue either.