The problem with presenting full-screen is that Zoom (or whatever videoconference app you're using) will capture your whole screen, including any black space outside the bounds of the slide. That extra space will tend to make your slides smaller on viewers' screens as Zoom makes sure to include every black pixel.
If your deck is in the same aspect ratio as your screen (or vice versa), then it works out; 16:9 deck on a 16:9 screen has no wasted space around it. But if your screen is 16:10 or something, a 16:9 screen will get letterboxed (black above and below); if your deck is 4:3 on most modern screens, it'll get pillarboxed (black to the left and right).
The inspiration for this post's title is that people may then tell you “please take your presentation full-screen”, which isn't helpful because it already is. Being in full-screen and sharing the whole screen is the cause of this situation, not a solution to it.
Another problem is that in order to go full-screen and share your screen, you have to do those things the other way around: Share your screen and everything on it, then hit the Present button. There's no opportunity to get the presentation into full-screen and then start sharing.
There are three solutions:
One, at least in Zoom, is that it has a “share from file” option in the Share dialog, and one of its options is to share from a PowerPoint file (“slides as background”). I haven't tried this and can't speak to how well it works. It's probably fine as long as your deck is not too fancy (basic rich text+images on flat or image backgrounds, no videos or fancy animations). Note that your video feed will be positioned in a corner of the slide, so either design an unsafe zone into your deck (and then move the video rectangle to there) or turn off your camera during the presentation.
Another option that Zoom has is “portion of screen”. This gives you a rectangular frame that you can position and size around the slide, either with the presentation full-screened or in a window (but you should still at least go into play mode). Viewers will see what's within the frame but nothing outside it. Note that this captures the screen just like a full-screen share does, so anything that comes up in front of the slide will show up (toasts/notifications, alerts, etc.). Your cursor will also likely be visible, so park it outside the boundary.
An advantage of “portion of screen” is that you should be able to set it up beforehand (including configuring both the capture frame and the video frame, and having the presentation open and in play/presentation mode and on the first slide) in a meeting of your own, then have it ready when it's time for you to present for real.
The last resort is to present your deck in a window rather than full-screen. This is most feasible with Google Sheets, because you can share the browser window. PDFs can also work (again for sufficiently basic decks). You'll have some chrome visible, but it's likely preferable to having your deck be one-third of your ultra-wide screen.