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Thank you for the kind words.

I have been meaning to answer this for some time, but putting it off because the actual answer is huge and I was intimidated at the thought of trying to come up with something a bit more concise.  

First of all, read Pat Wrede did an amazing write-up of world-building questions that I recommend to everyone.  Whether you are writing science fiction or fantasy, these are going to highlight areas of your world you haven’t considered.  Not all might apply to you, but it will give you a good jumping off place.

A lot of world-building in a fantasy setting is knowing how things work (or worked) in our world.  How big is the population of a city?  Of a high school?  Of a small town?  How big was one a hundred years ago?  How fast does a horse move when being ridden for an entire day?  How much food is needed for an army of ten thousand and where, historically, did that food come from?  How were taxes paid?  How did revolts happen?  What happens to people when they are isolated from others?  How do people behave when they have absolute power?  How did the printing press change the world?  How did salt?

What makes a world feel real is its resonance with the world we know.  Do your research and find those relationships.

The other thing that makes a world feel real is its people.  No matter how much technology and magic you add, people will continue to behave the way that people behave — make sure your people have real human motivations that make sense, no matter how strange.

And lastly, make interesting choices.  What makes a big world work exceedingly well is lots of potential stories.  Seed your world with them.

And once you’re done with that, my personal advice is to talk it through with other people. Let your friends and critique partners ask you the tough questions, write them down, and start figuring out the answers.

Hope that helps!