I guess you're writing about the Android build which is obviously a tiny fraction of the other build in size. To explain it in an easy way, a common practice of Android games is to release, initially, a launcher and then download the latest version of the assets. This is mostly due to Google's policy when handling game on the Google Play as Google handle games larger than 125MB differently than games smaller (but it doesn't count post-download size). Larger games are all packed in a single list while the rest is shown in the genres properly. Considering the low visibility of 99.99% of the games on the Google Play store, the last thing any publisher want is their game to also be tied to a single less desirable list.
Even if the APK on ichi isn't restricted like Google, making a version that has everything from the get-go would mean building a different version of the game just for ichi and that's a waste of time.
