Expansive can be used in a few different ways... Some people have an expansive personality, which means they are open and generous and accepting and have a wide understanding of the world, people, how things work i.e. there is an openness and spaciousness in who and how they are You can also use "expansive"to describe the wide, open, blue sky or a vast, dark night sky or how it feels to be on a boat in the middle of the ocean with no land in sight ... the feeling is wide, open, expanding, not limited... Extensive, on the other hand, is used to describe something that has a lot of details, was done completely, thoroughly... The professor has extensive knowledge of her field. She has studied, research and published extensively in her field.

They are synonyms. Synonyms ex·ten·sive adj. 1. Large in extent, range, or amount. 2. Of or relating to the cultivation of vast areas of land with a minimum of labor or expense. ex·pan·sive adj. 1. Capable of expanding or tending to expand. 2. Broad in size or extent; comprehensive: expansive police powers. 3. Open and communicative; talkative or effusive: Wine made the guest expansive. 4. Grand in scale: an expansive lifestyle. www.thefreedictionary.com