This issue concerns PDF Studio 8 only and is fixed in PDF Studio 8.4 and over.
In PDF Studio 8, an issue was introduced with cropping: Attempting to crop a document with more than one page size will result in all of the pages being the same size.
PDF Studio 8.4 will solve this issue as well as greatly improve the cropping process for all documents. We will show borders on the document to indicate where the crop will occur, and all crops will be relative to their own page size.