This is going to sound kind of crazy, but I would argue that Le Mort d’Artur probably had one of the biggest impacts on the world of any work of fiction (if you exclude the Bible and other holy books) and I think it would be classified as fantasy if written today though it’s best known as a classical romance

The Arthurian legends became so well known that by the 13th century they were indistinguishable from actual history and were even relied upon by English kings to justify the invasion of other countries- and not just in the crusades. Edward I (hammer of the scots) relied on these stories to justify to the Pope invading Scotland and other parts of Britain (and ultimately Ireland too). Down the road, even once Arthur was recognized as just a legend, Victorians still relied upon the mythology to justify colonial expansion as part of their culture.

More recently you could probably look to Dickens as being a cornerstone for part of a larger cultural revolution as well as creating new genres of literature and he was big even in his own time. But I don’t think that’s exactly what you mean.

And then there are the Russians. again, they’re not fantasy but Russian literature in the 20th century was seriously groundbreaking And still has massive impacts on many modern writers.

I guess the question is “what do you mean by big?” Read/known by a large part of the population? Having an impact on culture? Longevity? Or just how many $ they earned?