If the signature is smaller than the script, the writer does not expect recognition of their worth.
It may be this is a deliberate holding back, a defensive posture.
Complete legibility shows open and straightforward social attitudes. Legible script, illegible signature: the writer feels what is said is of more importance than his or her true identity, and is hiding his or her true self.
Small signature: the writer expects little esteem from other people.
This may imply a high degree of self-motivation and disregard for what people think, but is more likely to be a sign of low self-confidence.
Your signature is central to your identity, yet you sign your name thousands of times in your life, so often that writing it becomes a completely automatic, mechanical act.
The signature is a stylized form of writing: it shows how you want the world to percieve you.