Redact black & white PNG image in Paint 3D, saved file is 250% bigger

I don't think it's a fundamental limitation. Turning portions of the pixel array into uniform black should allow for smaller PNG files because the squares are all uniform black rather than a mixture of black and white, i.e., there is more entropy, and run-length encoding should be more efficient. This is specifically a fault of Paint 3D.

AFTERNOTE: My reference to run length encoding was overly simplistic, I found out afterward by googling.

Adding a few small black boxes shouldn't amplify the size by 250%, even if multiple layers are maintained. But I've never heard of PNG maintaining multiple layers other than colour planes and a transparency "alpha" channel. Are you certain that your explanation applies?

I tried "Paint", but it has no redaction feature like drawing obfuscating rectangles. I do not want to do this in Powerpoint and save back to PNG because it tends to inflate image file sizes grossly in addition to blurring the image. As far as I know, neither tool allows the specification of black-and-white image files.

P.S. The very question is about what program will do the job without gross inflation of file size (and I should add here, losslessly)

I fear that it may require some less mainstream and harder-to-use programs like GIMP. You basically have to become an expert to redact black-and-white scans.