That's a neat project!

When generating huge numbers of png images, it's worth it to tweak the compression settings. Run png optimizers like optipng/pngcrush on your output, and use the settings it finds.

LodePNG is easy to use, but doesn't let you tweak as many options as libpng.

The tiles I've inspected are 15-30% larger than necessary-- besides using sub-optimal compression heuristics, they include unnecessary metadata (creation/modification time, aspect ratio).