As per your explanation there should be little difference of size between images, But I upload now an Image with size 3.3KB and after uploading its 42.41 KB,
I don’t know where from its getting but the image URL is something like: wp-content/plugins/wp-portfolio/cache//e3341c8aded40f6f565bb14d3caac3c8_036eab42571c65137759a104a340a182.png
I wonder why cannot the same image can be used as Portfolio why need cache? or why there is no option to disable it and used the default image uploaded, because the image I am uploading is already compressed to enough.