You've seen these dresses. They have a knit bodice, tank-top style, generally black, and then there is a brightly colored print or solid skirt. The waist is high, the hemline is high, and the tailoring was done by someone who was high.
I think someone is under the impression that the high waist and hemline give the illusion of longer legs. They might. But only at the cost of the illusion of morbid obesity. I have seen thin girls in these things. They don't look thin. Girls who are already not thin look less thin. This is not a good fashion choice!
Seriously. These dresses are awful. Sometimes they are paired with leggings. A newsflash, ladies: no garment is improved by the addition of leggings.