Very strange ppi of png
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..
Very strange ppi of png
===

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..