Why are PNG images bigger in file size on the (S3) server?
I agree with Brad's comment, this is very similar to those two other threads.

That being said, here is an important quote which is buried on one of those threads...


  When an image is first uploaded, Craft will "sanitize" the image for any malicious code by re-saving the image out to a new file. It jumps through some pretty significant to ensure that the sanitized image is as close to possible as the originally uploaded image in both quality and file size, which usually explains the differences people see in file sizes.


I believe this is what the OP is experiencing.