Convert Image - JPG and PNG work differently

I use the Convert Images feature a lot to reduce the size of images that have been uploaded to my site. They are a mix of JPG and PNG images.

When I do the conversion, I don't change the type (that is, JPG stays JPG, PNG stays PNG), just make the max size 800x800 (retaining aspect ratio). For the JPGs, DO only changes the ones that are bigger than either of the two dimensions, making them smaller, which is what I want. But for PNG, it makes ALL images fit the specified size, making smaller images much too big.

Can you make the Convert Images feature work the same for PNG as it does for JPG?

What are the commands you are using currently?

(I'm assuming you're using custom commands and not the Convert Images GUI, since the GUI always resizes things if told to resize, as far as I can tell.)