Most of the bulk can come from adding immersive shows which are longer than tells, or possibly adding subplots to make the story more complex.
Reading Mistborn again, and really appreciating how we get three interwoven storylines for Kelsier: His effort to overthrow the Lord Ruler, his relational mentoring of Vin, and his angst and backstory over his dead wife, Mare. He has these different sort of narrative states he exists within. He feels real, as a result, and that's just one character. Vin is the other viewpoint and she's got multiple things going on as well. Sazed is not a viewpoint character at all but is fully fleshed out. The named cast is probably close to thirty at least, when all is said and done.
I can easily imagine the first draft of a fat novel like this being a third the length.