Larger in what way?
Page dimensions are saved in the file and don't change on their own. What do you see, and what does she?
When you choose File > Document Setup in InDesign on both Mac and PC, you're saying the "physical" page size changes? That shouldn't be possible.
- If your screens are different resolutions, the size may look bigger or smaller when you fit the page in window. However; the document dimensions should be in tact.
- If your Supervisor is in Preview or Bleed Preview in InDesign (View > Screen Mode > Normal), dimensions may also look different (but my first guess is you are noticing a resolution difference between Mac & PC).
- Are you PDFing the file? If you include Bleed and Marks in the PDF file, Acrobat will report a different page size. If you turn off Marks and Bleed, Acrobat will show the trim size (or InDesign document dimension).
Can you give us any more detail?