1600x900 keeps the exact same 16:9 shape as full 1920x1080, just with about 30 percent fewer total pixels. On a laptop screen, inside a presentation slide, or in a social feed, that difference is rarely visible to the eye, since most of those viewing contexts were never going to render every pixel of a full HD source at 1:1 in the first place.
Where the smaller pixel count starts to show
The gap becomes noticeable in a couple of specific situations, viewing the image at full size on a large 4K or retina display, where 1600x900 has to stretch across more physical pixels than it actually contains, or printing it at a large physical size, where fewer source pixels per inch shows up as softness. For a screen-only presentation or a social post, neither of those situations usually comes up.
Platforms revise these numbers more often than you'd expect
X's own preferred in-feed image size shifted from 1200x675 to 1600x900 as its feed display changed. An asset built to the older spec still uploads and displays fine, just not quite as sharp as one built for the current guidance. It's worth rechecking a platform's documented size every so often rather than assuming last year's export settings are still the right target.
Matching the format to what's actually in the image
For a photo-based banner or presentation background, JPEG or WebP keeps the file light without a visible quality hit. For a slide or dashboard graphic built mostly from text, icons or flat color blocks, PNG holds those sharp edges cleanly where JPEG's compression would introduce blur around the letters.