why png size doesn't change after using http gzip compression

Question

I use following .htaccess to set gzip compression:

AddOutputFilterByType DEFLATE text/html image/png image/jpeg text/css text/javascript

Pl

Answer 1

PNG is already a compressed data format. Compressing it with GZIP is not likely to decrease the size, and can in fact make it larger.

I'm surprised you're seeing benefits when GZIP-ing JPGs, as they are also compressed.

See here for Google's tips on using GZIP. They recommend not applying it to images.