Just found this issue, and look forward to getting advice from you:
For the product image uploaded via admin page, when it shows in the product page, or at home page, the source codes shows that it is loaded from a path like this:
The actual product image has only dimension of 400x400, the cached image from the above path has dimension of 600x480.
The resized image (to 600x480) has a much bigger size, which causes slow site speed.
The list.phtml and media.phtml only resize the image to 135, 56, and 265.
Can anyone share your idea why this is happening? Thanks!