Small is good
Many games are now downloaded and patched online and the size of your game can be a download cost for your players. Size optimization is when you try to get the game package as small as possible. It can be the size of the download, storage on the customers computer or the save file. The progress is measured in bytes.
Profile - Bytes
IOS Over-the-Air App Store limit: 100MB.
Google Play APK limit: 50MB
Make a pass over all assets and make all of them are still in used. Often many old expired ones get left in the build.
Make a tool to list all assets by type and size and verify that they do not use more memory then they need for the place they have in the game.
Common assets to look at are textures and sound.