Why so many forms stop at 20KB
The 20KB signature limit is a fossil. It comes from application systems designed when a recruitment board might receive hundreds of thousands of submissions over a dial-up-era connection budget, and storage was costed per megabyte. A 20KB cap on a scanned signature was a reasonable engineering compromise then, and the validation rule has simply never been revisited.
The result is that a rule written for one era is now enforced against phone cameras that produce four-megabyte files by default — roughly two hundred times the allowance. That is the whole problem: nothing about your signature needs four megabytes, but nothing in the pipeline between your camera and the upload box knows that.
Scanning a signature so it survives the squeeze
The single biggest factor in whether a signature makes it under 20KB legibly is how much of the frame the signature occupies. A signature that fills the frame gets the whole budget spent on ink. The same signature photographed in the middle of a sheet of paper is asking the encoder to spend most of its bytes describing the paper around it, and paper is not as cheap to encode as you might expect — its texture and the subtle lighting gradient across it are exactly the kind of low-contrast noise that compresses badly.
Sign in dark ink on plain white unlined paper, photograph or scan it straight-on in even light with no shadow falling across the page, and crop tightly to the ink before you compress. Those four things together will usually get you under 20KB with quality to spare, and they matter far more than any setting in any tool.
Avoid photographing a signature at an angle. Beyond the obvious distortion, an angled shot produces a lighting gradient across the page that the encoder must spend bytes reproducing, and it tends to throw part of the signature slightly out of focus — blur that compression will then amplify.
Transparency, and the black box problem
A signature exported from a drawing app or removed from its background is usually a PNG with a transparent background. JPEG has no concept of transparency, so when such a file is converted without care, every transparent pixel is written as black — and because the ink is often black too, the result is a solid black rectangle that hides the signature completely.
It is a common failure, it looks like the tool has destroyed the file, and it catches people out precisely because the image looked fine a moment earlier in their file browser. This tool paints a white background in before encoding, which is both what avoids the problem and what the form expects to see anyway. If you have used another tool and got a black box, this is what happened.
If it still won't fit
Crop harder first — it is nearly always the answer, and it costs nothing. After that, check whether you are working from an already-compressed copy: re-compressing a JPEG that has been through the process before compounds the artifacts, so start from the original scan if you still have it.
If your signature is genuinely large and intricate, consider whether the form really wants 20KB for the signature specifically. Many portals publish separate limits — commonly 20KB for a signature, 50KB for a photograph — and the two boxes are easy to mix up. Uploading a photograph against the signature limit is a much more common cause of trouble than an unusually detailed signature.