This is a 1 command creation that improves slimes! Slimes will start as babies, then grow over time! Bigger slimes are slower than smaller slimes. It is small so that it has survival compatibility.
To install this command creation, give yourself a command block with /give @p command_block and paste the command into the command block. Then activate the command block with something.

Download Link: http://pastebin.com/UXJJsk5e