Automatic Resizing of Form Field to Fit Space (NOT of Text)

I do a lot of preparation of documents for digital signature for my company. I have noticed that if I am working on a document that has a defined space for a form field to be entered (such as an empty box), when I enter that field into the space, it will often automatically resize the field to fit the whole defined space. (Ex. Entering a digital signature field into an empty box on the document, the field auto resizes to fill the dimensions of the box). However, this feature works only sometimes. Even within the same document, it seems to sometimes hold true and sometimes not.

Can anyone shed any light on how to make this feature apply all of the time instead of only some of the time? What triggers Acrobat to recognize 'boundary boxes' when you put a form field into the space and what/how can I alter these prepared documents to encourage Acrobat to recognize the spaces and auto-resize every time instead of occasionally?

(NOTE: I am NOT asking about text resizing. I understand how this works. My question is regarding the auto-resizing of the form field itself when initially dragged onto the document. I experience/use this most with digital signature fields and date fields).

Thanks to anyone who can help!