is there a way to force the filetype of generate cache images to a specific filetype? I've noticed that while the file sizes of jpg files are fine, when i upload png files the size is un-proportionally large for the image dimensions.

Maybe some hints on which core functions process the cache to be able to force jpg images?

No, the file format follows the original image. Of course png files are bigger because jpg compression is lossy and png - especially png24 - not so.