Anybody knows what hefty prose is and add an example please. I don't know if I am using that style in my science fiction and fantasy stories.
I think you're asking us to define something that is subjective and far too open to interpretation. Hefty means 'large and heavy / done with vigour and force' so you can interpret it from that.
I honestly have no idea. Maybe someone else will come along and be able to interpret your question better. Hefty is hefty, and prose is prose. I really can't say what is meant by 'hefty prose', other than to say that by definition it means 'heavy/large/vigorous/forceful prose'.
I've never heard prose described as hefty before. It's certainly not a style, either. Maybe you're thinking of pieces with long sentences and paragraphs?