Consider that volume is a measure of size. That will help you understand that voluminous refers to something very large in size or extent.
Suppose instead of calling a book a book, you called it a volume. Suddenly, it seems to be part of a much bigger thing. And what if it were only Volume I, with 23 more volumes to go? Even bigger, right? In fact, it's more than big; it’s voluminous. Voluminous describes very large things. The flouncy skirt of a wedding dress? Voluminous. Media coverage of some movie star's divorce? Also voluminous. The word voluminous describes something ample, extensive, and just plain huge.