Why Image compression increases size of image?

[g-abhishek]

I am trying to compress the PNG image using imagemin and imagemin-pngquant, after applying the transformation with different quality level, getting different sizes of image

See for quality between 0, 1 => 1848433. and for quality between 0, 0.8 => 2051244. for lower quality image size is greater than the higher quality.

e.g
input size 1677003 bytes
quality: [0, 0.1] => 1227453
quality: [0, 0.5] => 1662248
quality: [0, 0.7] => 1886730
quality: [0, 0.8] => 2051244
quality: [0, 0.9] => 2018915
quality: [0, 0.95] => 1995294
quality: [0, 1] => 1848433

Sample code:
let res = await imagemin.buffer(buffer, { plugins: [ imageminPngquant({ quality: [0, 1], }), ], });

Sample image:

[ikitty]

got the same problem. cannot understand the 'quality' variable.