Export As PNG Transparency vs. Smaller File (8-bit)

I want to export the PSD of my digital art as a high-quality, lossless PNG file, to keep as my archive copy.  The art piece was created in the sRGB color space, does not have any transparent areas that I need to preserve, and 8-bits of color is fine with me.  

However, I don't understand why the "File Settings - Format: PNG - Smaller File (8-bit)" option produces a file that is only 858 KB when the "Transparency" option is 3 MB, and the "JPG" with "Quality: 7 High" is 1.9 MB.  Transparent or opaque cannot be the only difference between these two results.  858 KB sounds like a crummy, low-resolution image for the web!

Should I forget about the mystery meat smaller file option and stick with Transparency?