Signature font size changes when reply – not new message (Outlook 2011)

When I create a new email all is fine.  However when I reply to select emails the font size of my signature increases from 15pt to 16pt.  I cant pick the pattern here as to what sort of email it does this for.  All are Outlook for Windows originated.  This happens always for the same senders.  None of which are sending in plain text.

Any thoughts?

I have the same result when ticking or unticking " When replying or forwarding, use the format of the original message"

Im running Outlook for Mac 2011.