If it's anything like Xenoblade on Wii it will have a metric kiloton of VA dialogue, so that will account for a lot of its size before we even get to graphical assets.
Also, the soundtrack takes up 4 CDs: http://www.nintendolife.com/news/2015/04/xenoblade_chronicles_xs_soundtrack_is_so_epic_it_needs_4_cds_to_contain_it








