A limit that rarely costs quality
One megabyte is generous for a screen picture. A typical photo is 3 to 10 MB because cameras save at a very high quality with a lot of invisible detail. Getting such a photo to 1 MB is mostly a matter of removing that invisible detail, which is what the quality search does. The picture usually keeps its pixels, and at normal size it looks the same.
If a photo is exceptionally detailed, such as a close shot of foliage or a crowd, the quality needed to fit may start to show, and the tool then also reduces the pixel size slightly. You can see the final size in the list.
Scans, documents and phone photos
Scanned pages at high resolution can be several megabytes each. Compressed to 1 MB, a page stays readable if the scan was clean. Text needs sharp edges, so prefer JPG or WebP at the quality the tool chooses over making the scan smaller in pixels. For many pages, consider combining them into a PDF: Merge PDF joins files and Compress PDF reduces the result further.
In our tests a 3 megapixel photo of 1.5 MB came out at 458 KB at quality 83 with its full size, and a 6 megapixel photo of 3.0 MB came out at 830 KB at quality 77, also at full size, so the limit cost no pixels at all. A modern phone photo is bigger still, so expect the tool to lower the quality a little more or reduce the pixels slightly. The note on each result says which.
How a megabyte is counted
Two systems are in use. In the first, a kilobyte is 1,000 bytes and a megabyte 1,000,000, which is how storage makers, macOS and most web forms write it. In the second a kilobyte is 1,024 bytes, so a megabyte is 1,048,576, which is how Windows Explorer shows sizes. A form that says 1 MB may mean either. This tool counts the smaller one, so a result that is under 1,000,000 bytes is under the limit in both systems, at the cost of a few percent of room. The sizes in the list use the same counting, so what you read there is what the limit saw.
Tickets, forms and shared drives
Support systems, application forms and shared drives often cap a single file at 1 MB. Compressing to the limit means the screenshot or photo goes through the first time. If a service counts the total of several files, divide the limit by the number of files and use the smaller page, for example 500 KB.
Compress a copy, not your only file. A compressed picture cannot be turned back into the original, and anything you will edit or print later deserves the full resolution.
When a picture is already under 1 MB
A picture that already fits is not stretched up to the limit. If compressing it at the Balanced level would take off a worthwhile amount, you get that smaller file. If it would not, or if the picture is already about as compressed as Balanced would make it, the original comes back unchanged with a note under it, and it keeps its own name. A picture in the same format is not returned bigger than it came, except in the rare case where it has to be saved again to take out a location, and then the note says so.