Thomas Monson's message in this month's Ensign is about serving in our church callings which he equates with loving and serving as Jesus did and taking responsibility instead of seeking to be comfortable.
That article and its sidebar, of course, use quotes that refer to "magnifying your calling". I've sat in on classes where teacher and students have discussed what that word "magnify" means or doesn't mean in that context where they have had a bit of a struggle creating a definitive definition.
Today I found this:
Magnify: late 14th century, "To speak or act for the glory or honor (of someone or something)," From O. French magnafier, from Latin magnificare, "esteem greatly, extol", from magnificus, "splendid". Meaning use of a telescope or microscope is first attested 1660s.
It is relevant to note that the King James Version of the Bible was finished in 1611.
So, perhaps, if one is called by the Lord to do some work in his Kingdom, then to magnify one's calling mostly just has to do with acting in that calling in a way that honors the divine nature of that calling thereby speaking and acting for the glory or honor of the Lord; speaking and acting in ways that reflect his way of loving and serving and that help turn others hearts to honor him, not making that calling bigger or clearer or more visible or detailed as later definitions of the word came to mean.
That makes sense to me.