"Signature size should be between 10 KB to 20 KB" — usually the minimum, not the maximum
A signature is a few dark strokes on white paper, which compresses to almost nothing. This error is far more often about the floor than the ceiling.
A signature is the easiest thing in the world for a JPEG to store: a large flat white area and a few thin dark lines. Once it has been cropped and set to 140 x 60 pixels, it very commonly lands at 4 or 5 KB — comfortably under a 10 KB floor, and rejected for it.
Check which end first
If your signature file is in single digits of KB, you are failing the minimum, and compressing it further is the opposite of the fix. If it is over 20 KB, you have probably not resized it yet and are uploading a full phone photograph of a page.
Too small: pad it, do not degrade it
There is no honest way to make a picture of a signature need more kilobytes without making it worse. What you can do is pad the file. A JPEG may contain comment segments that every decoder skips over, so bytes can be added while the image stays byte-for-byte identical. Set the minimum in the signature resizer and it pads automatically when the result comes in under the floor. The file stays a valid JPEG.
Too large: crop close, then size
Most oversized signature files are a photograph of a whole sheet of A4 with a signature somewhere in the middle. Crop tightly to the strokes first. This matters for more than the file size: a signature box is wide and short, usually 140 x 60, and an uncropped page squeezed into that shape leaves your signature tiny in a sea of white.
Keep it looking like your signature
Whitening the paper is optional and off by default here, because a scan that is already clean comes back worse for it. If your photograph came out grey or shadowed, turn it on and it lifts the paper while keeping the soft edges of the pen stroke. If it looks fine as it is, leave it alone — the output will match what you uploaded.
Questions
Why is my signature only 5 KB?
Because there is very little in it. White paper and thin strokes compress extremely well. That is normal and is exactly why the minimum catches so many people.
Can I make the signature bigger to increase the file size?
Only if the form has not fixed the pixel dimensions, and usually it has. Padding the file is the correct fix when the dimensions are already set.
Should I sign on lined paper?
No. Use plain white unlined paper and a black or dark blue pen. Lines become part of the picture and can be read as part of the signature.
Does the portal compare this to my exam-hall signature?
Yes, that is the point of collecting it. Upload the signature you actually use, not a neater version you have practised.