And if you run it through https://tinypng.com it should get down even smaller. PNG compresses surprisingly well when it’s basically two colors + alpha.
Photoshop: 6 705 bytes, 128 colors, 1-bit alpha only
TinyPNG: 6 179 bytes, 92 colors
Optimage: 5 761 bytes, 154 colors
TinyPNG + Optimage (lossless): 5 226 bytes, 92 colors
TinyPNG tends to use fewer colors. But its compressor is pretty suboptimal. I could reduce the image further by 15.4% with the tool I'm making (http://getoptimage.com).
I see 3 colors (blue, white, transparent).
The original PNG icon was very clearly suboptimal at 20 225 bytes...
$ curl -OL https://i.imgur.com/dHIaRh1.png
$ identify dHIaRh1.png
dHIaRh1.png PNG 512x512 512x512+0+0 8-bit sRGB 3545B 0.000u 0:00.000You can also losslessly compress it down to 2 803 bytes: https://imgur.com/a/SunI7.