Very strange ppi of png [closed]

Question

I have some png files, and their ppi were 300.

Now I've change the ppi to 72, but the size of the pictures even became larger!

For example, a.png was 190k, 300ppi,

now it becomes 400+k, 72ppi, and it is always 320*460,

what's happening, how should I do?

I was meant to reduce the pictures's size..

Answer 1

ppi stands for 'pixels per inch'.

The equation for image size is NoOfPixels = NoOfInches * PixelsPerInch

Or NoOfInches = NoOfPixels/PixelsPerInch

So reducing the ppi will increase the 'physical' size of the image.

To reduce the size of the image you need to squash more pixels into each image of 'physical' space, so increase the ppi