Hard to narrow this down. If the size doesn't change at all when you adjust the pots, I'd start by metering them to make sure they aren't shorted. If it does change size, but not enough to get the whole image on the display, then I agree with Slackmoe.
If you have a spare boardset or display, swap them to isolate the fault.
Be careful, if your game PCB is putting out more X/Y voltage than spec, you can damage your monitor. Spec says:
Signal Inputs (at Maximum Deflection)
"X" horizontal: 16 volts P/P (±8 volts).
"Y" vertical: 12 volts P/P (±6 volts).
"Z" beam drive: 4.0 volts maximum brightness, 1.0 volt black level.
I've recently seen this exact thing, Turned out to be a bad 7915 regulator. Instead of -15 it was putting out -22 or rail. which caused the image size to be huge and non adjustable. Which is why I asked about the voltages. Could be something as over voltage causing excessive swing.