File size and image size are two different numbers
People say image size for two things. File size is how much storage the file takes, measured in bytes, KB or MB, and it is what upload forms, email attachments and job or exam portals limit. Pixel size is the width and height, such as 1920 × 1080, and it decides how much detail the picture holds and how large it can be shown or printed.
This checker headlines the file size and lists the exact byte count next to it, then shows the dimensions, megapixels, format, MIME type, DPI and EXIF data underneath. Two files with identical dimensions can differ hugely in file size, because the format, the compression level and any embedded metadata all add or remove bytes.
How KB and MB are counted
Here 1 KB is 1,024 bytes and 1 MB is 1,024 KB, the same convention Windows uses. Some websites and macOS count 1 KB as 1,000 bytes instead, so the same file can show slightly different numbers in different places.
When a limit is tight, compare the exact bytes. A form that allows 50 KB means either 51,200 bytes or 50,000 bytes depending on how it counts. A 50,500 byte file shows as 49 KB here and passes the first kind of limit, but fails the second. If you are close to the line, aim a little under it.
Why is my image file so large?
Modern phone cameras capture 12 megapixels or more, and every pixel costs bytes. Photos saved as PNG are much bigger than the same photo as JPG or WebP, because PNG is lossless. Saving at maximum quality, or keeping a large embedded thumbnail and metadata, adds more on top.
To bring a file down, compress it to a target such as 100 KB, reduce its pixel dimensions with the resizer, or convert a PNG photo to JPG or WebP. Check the result here again before you upload it.