There could be a number of reasons for the disparity, and it can be difficult to diagnose without looking at your set-up or better understand what exactly you're seeing and what you expect to see
That said, I've found in most cases it's a connection issue, specifically the display monitor (projector) has a different resolution than the desktop. If you've set your desktop to simply mirror the screens, you may end up with a projector display "floating" in space, rather than filling the whole screen as it appears on the desktop.
- Forcing the resolution of the projector to match that of your laptop display (right-click and choose Screen resolution) should allow the mirror option to work without dead space. This may distort the view, so it may not be optimal
Perhaps a better remedy is making sure your computer recognizes the projector as a separate monitor, then drag-and-drop your viz onto that screen. This allows you to utilize the full potential of the projector, though it also means you may not have your viz on the laptop in front of you.
In my years of working with monitors, I've yet to find a silver bullet; I remain convinced that they must be manufactured with some brand of black magic (as tricks I've used to make them work one week seem to evaporate the next). Sometimes it's as simple as closing the lid of my laptop, and the monitor comes alive as the primary screen-- BINGO!
Whatever you do-- experiment a little before your big presentation. Good luck to you!
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