I was able to trim over 1 GB of data from the Pics folder by optimizing the PNG images with Oxipng. (Oxipng is a Linux utility, but there is a windows binary available)
Broadly, Oxipng does three things: 1. It strips useless metadata from the files. 2. It changes the color values of fully transparent pixels to something more easily compressible. 3. It tests each of the compression algorithms supported by the PNG format (all lossless) to see which one is most effective for each image.
The command line switches I used was: oxipng -varo6 --strip all Pics
(Pics being the name of the directory I directed the utility at) It took just under 30 mins to process all 7579 (!!) png images on my machine, resulting in a total install size of 2.3 GB.
Unfortunately, it doesn't seem to result in a significantly smaller archive. The zip I created of the smaller install is only about 70 MB smaller than the zip downloadable from itch.io. It seems zip is pretty good at compressing unoptimized pngs.
In any case, it might be worth while to look into this utility if you're interested in a smaller overall install size.
Thx for the great game!