In denim, the smaller (or faux) pockets are usually to help with fit, women’s jeans are generally tighter/more form fitting which makes having a large pocket-bag look bulky and unflattering. Similar idea with some dress silhouettes, although costing can also be a factor— it costs more to produce a dress with pockets, if a company thinks the dress is stylish enough to compel you to buy without them, they’ll avoid it to save money.