We're using PDFBox (as backend used by Esig DSS) for digital signing of documents. We've found out that visible signature has different look on Mac Preview app than other PDF viewers. After some investigation we suspect that BBox-es for appearance of signature is not filled correctly by PDFBox. The values are hard coded as [0, 0, 100, 50]. Also transformation matrix for translating from form XObject coordinate system into default one has value (1, 0, 0, 1, 0, 0) -> no scaling between default user coordinate system and form XObject coordinate system.
Therefore I think that the Mac Preview App draws signature with dimensions 100x50 because no transformation were done.
I've attached patch which solves this issue for us. The signature is shown correctly in all PDF viewers now. Maybe you can take a look on it and integrate it into your code base, or propose better solution.