I generate my PNG splash screens using Imagemagick, which seems to do a bad job of compression by default. I noticed that the PGB does a pretty good job of compressing them for the Android builds, but not for the IOS builds. What does PBG use for compressing the Android images?
Oops I noticed that you do a pretty good job on IOS content as well. I'd still like to know what compression PBG uses.
Are you referring to the stock splash screens being nicely compressed?
Note that neither PGB nor the CLI apply any additional compression beyond what you may have already applied on your own splash screens.
It might help to see your splash screen (before & after) along with the settings you are using to see what problems you're having there.
As an example, the Android splash screens and icons i uploaded (even with compression) amount to 5.1MB. In the resulting android package they are less than 900KB.