This is a term used in document signing.

Small sign refers to "small sign". Generally, the signer draws a signature similar to a symbol on the lower right (or left) corner of each page according to his or her own writing style. Note that this small signature has a legal basis. In order to indicate that this page has been incorporated into the official document, no changes may be made.

The "big signature" refers to the last signature place of the official document, including the time and place.