reducing dimension of an icon with Preview
There are multiple algorithm to optimize the size of PNG files.
Preview is known to not be not optimal on that point. 

If you want to optimize the size of your PNG, there are multiple tools available on the web. 

Just by randomly searching on google :


https://pngquant.org/
https://imageoptim.com/
http://www.gingerbeardman.com/pngenie/