The phrase "voluminous top" refers to a clothing item, particularly a shirt or blouse, that is loose-fitting and has a lot of fabric or material, making it appear larger and more spacious. Full definition
The vertical striped pants elongate your legs which gives some height to your outfit = perfect opportunity to match with voluminous tops etc.. (thefashionfolks.com)
If you're pear shaped, you'll be told to go for more voluminous tops to even out «the volume». (thefashionfolks.com)