Hi, I'm Matt and would like to help you (23 years an IT developer and a webmaster, even longer than that using and researching digital image formats including fractal compression and wavelets).
While I'm in other "waters" now and have to do other tasks for living (I graduated in MPEG-7, ISO specifications for semantically describing multimedia with XML) I believe I can work on the code. PNG has its freely available code so this is the first I want to ask - what do you use right now?
Now for my opinion - I used to work with LEADTools, the fastest (commercial) image compression library few years back. That is probably not an option for you - due to licencing price - yet it offers easy path to incorporate image support in any Windows application.
Either way,, looking forward to discuss. Sure, PNG-8 images should be much less in size compared to its PNG-24 saved same image. So maybe your library or calling the functions have some issues that needs to be researched and fixed. And generally - I am more Delphi an Java developer so have mostly mentored C(++/#) developers in the past. Though any programming language is not an issue if functions to use (other) code are required to fix, change and optimize.
Regards,
Matt