I’m not sure how familiar you are with the inner workings of PDF, so here are a couple of screenshots that will help explain.
This is all about Page Boxes.
Firstly you have the Media Box. This is always the largest Box, and if you think of a PDF as an electronic document, think of the Media Box as the paper that the document is on.
The you have the Crop Box. This performs a ‘soft crop’ so that Acrobat looks like the page is cropped, but in reality it isn’t the Crop Box just hides anything outside it.
You actually don’t need this for Print, it can be safely removed
So basically to achieve the result you want, you need to make the Media Box 20x20, enlarging from the middle so the content is not moved.
You then need to remove or edit the Crop Box as well, or the new size won’t show in Acrobat.
In PitStop, manually you can do it in the Inspector (in the panel shown in the Screenshots), or you can do it via a Global Change or an Action List (attached).