Resized images in cache ist 4 times larger

I have a big problem.
When i upload an image on modx then the cms create an image for the cache-folder.
Original image ist an PNG with 1240x400 px and 458 KB large
MODx cache image is also an PNG with 1240x400 px and 1'987 KB (4x larger!!)

In my slider on the header of the website i have 3 images... 3x 2Mb... this ist not normally...

What is the problem? Where can i solve this dilemma?

Thank you for answers

Do you use a image resize snippet like pThumb?
Then it isn't so wierd that this happens.

If you want help with this, then you have to tell us more details like modx version and the code/syntax that creates the image..