Hi @GrahameB
i'm ignoring all other replies and my answer is:
Papersizes are part of your plot driver, or stored in a pmp file, which are linked to plot driver.
Every Papersize definition have 6 properties, length & width + non-printable border left/right/top/bottom.
If enabled, AutoCAD trying to display your selected paper taking into account your print settings,
if enabled, AutoCAD also trying to display the printable area of your selected paper taking into account your print settings.
- - -
Now, the part i am missing in the other answers:
We are missing a senseful pagesetup, AutoCAD is missing them too - this is why your paper&borders doesn't look well.
Open PAGESETUP or PLOT, in my sample we start PLOT command.
Choose a printer, in my sample "DWG to PDF.pc3",
choose a papersize ANSI D
and all other settings, EXTENTS, Center,
press [APPLY to layout] and if you like, [OK] for a test pdf.
You can see: Your frame is big for this paperdefinition (doesn't fit into printable area).
Start command PLOT again and change ANSI D to ANSI expand D,
Use [APPLY ...] otherwise the changed setting is temporary (if you haven't turned on the automatic applying option).
and [OK] for the next Plot test.
NOW, you have a senseful pagesetup and your frames fits the choosed plotter-Papersize+settings.