But that sense of big was flexible. So, in some usages it went from meaning big to meaning too big. That sense of too big came to refer also to the elements that made something up and to the thing itself. So a very fat person might be called a “gross” person, or a very coarse fabric might be “gross.” This became equated with being generally distasteful, vulgar, or disgusting.
In another direction it retained its sense of largeness without the negative connotation. That morphed into gross (as opposed to net) being the largest measurement of the object with nothing taken away. It also appears to have been used by the french as a term to denote a “big” dozen, which came to be the term for 12 dozen.