It cannot be length alone, or a sonnet could not be protected by copyright.
"Ownership" of a written work, I believe, has to do with permission to have it printed, and the commercial value of what is printed, where and how it is printed. As a writer, that has been my concern. I write articles, essays, short stories and books and that is part of how I earn my living. No one else can "have" what I write, especially if they are making money from my writing or if their publication of it interferes with my right to gain from my work, without my permission.
It was different in Luther's time. In "Brand Luther: How an Unheralded Monk Turned His Small Town Into a Center of Publishing, Made Himself the Most Famous Man in Europe and Started the Protestant Reformation," by Andrew Pettigrew, we read how publishers rushed to print Luther's writings. Luther himself was fussy about which publishers he chose, preferring for a time the publisher in Lepizig rather than the one in Wittenberg because they did better printing and had better fonts. But he could not prevent any printer anywhere from printing and selling his work.