Unoptimized .pngs is the answer to your question. Even when I use older Photoshop algorithms, resaving the .pngs drops the file size significantly. And saving the 'non-transparent/full background' images as .jpgs can save even more space, but the backgrounds are a small percentage of the total number of sprites in SU. There are over 11,000 sprite images in SU Unity currently...
Sure, a number of these sprites are smaller body parts (heads, arms, eyes, lips, hair, etc.), not full images, but it all adds up!
Using optimization algorithms, or say webp, can drop the image size even further, with imperceptable loss of quality. If these were 'camera' images maybe, but since this is cartoon sprite art, the colors are a lot more uniform, so it can benefit greatly from optimization without perceptable loss in quality.
Regarding the SU Unity package, it is what it is, but as for the unofficial port, this IS something that is being addressed in the unofficial RenPy port!
So it's not really my place to criticize, but if the GS Games crew would just optimize their .pngs... that still wouldn't help with the bugs though!