Subsequent builds getting smaller.,Subsequent builds taking less space?

It seems that every build I make of my game, the physical space on my disk taken by the build is reduced (eg; build 1 takes 15MB but the next build after that only takes 14.5MB). This is despite me adding vast amounts of content between builds. Can anyone explain to me why this is?,It seems that every build of my game takes up less size on disk than the last, despite me adding vast amounts of new content between builds. Can anyone explain to me why this is?

@duckoutnow
If your project is working completely fine and not showing any error, I do not get why this should bother you.
It could be because of any Unity’s internal optimization technique which tries to club together similar files in the package.