A thumbnail's file size and pixel dimensions describe different things. File size tells you how much storage the encoded image uses; dimensions tell you how many pixels form its width and height.
Compare like with like
Two images with identical dimensions can have very different file sizes because of compression, detail, and file format. A busy photograph often needs more data than a simple flat graphic at a similar quality setting.
Diagnose the right problem
If an image looks blurry, inspect its dimensions and source quality. If it is too large to upload somewhere, check that destination's current file-size requirement and adjust the export. Reducing dimensions is one option, but it is not always the first or best one.
Inspect the export
After compression, open the new file and examine text edges, faces, and smooth gradients. Keep the original separately. A smaller file that damages the central message is not a useful optimization, even if it satisfies a storage limit.