Quality VS Quantity
I had previously optimized some of the enemies by down-scaling their animation sprite sheets. I was able to reduce the installation size by 1.2 GB. However, after play testing the game a few times, I started to realize that some of the game's graphics looked kind of bad. After improving the graphics a bit, the overall change increased the installation space by only 100 MB. This isn't bad, but there's still more optimization to come. (We're still down 1.1 GB from the original file size)

Gameplay Tweaks
I have continued tweaking the game play by adjusting various enemy stats (such as health and damage for example). I have moved some enemies around and added a few enemies here and there.