PDF dimensions bigger than document

Hi there…

Why is the exported PDF dimensions bigger than Omnigraffle? What I mean is…

I create an Omnigraffle PDF at A2
I export as PDF at 100%

If I open the PDF at 100% zoom, then put that next to Omnigraffle zoomed to 100% on my monitor, the PDF is way bigger than Omnigraffle.

Is it bigger because the DPI of the PDF is set to 300 (or at least larger than the dpi of my monitor)? I think that sort of makes sense…

Thanks