Clothes will look better on people with certain proportions. Most (male) runway models wear a size 40L (drop 8). I don't care what label suit you are wearing, it will always look better on someone who is a 40L.
At first, I agreed with everybody's argument that clothes just look aesthetically more pleasing on a slimmer person. However, I read this interesting article in the LA Times which made me question this premise. Here's part of the article: "Believe it or not, there already are sizing standards, though it's unclear who follows them. The standards, based on data gathered over nearly seven decades, continue to evolve but generally remain geared to the hourglass figure (think 36-26-36), which accounts for only 8% of women, said Istook, who has researched the female form. By her reckoning, nearly half of American women are rather rectangular (more like 41-34-42). The second-most-common shape is called the spoon. (Don't ask.) The first set of standards was devised in 1958 after catalog sellers such as Sears, Roebuck & Co. and Montgomery Ward, fed up with customers returning so many garments, asked for guidelines. They were based on a 1939 study conducted under a Federal Work Projects Administration grant, a major effort involving 58 painstaking measurements of nearly 15,000 volunteers across seven states. Unfortunately for future shoppers, the volunteers, who were measured with and without girdles, were not exactly representative. A disproportionate number were young, single city dwellers. Women of color were intentionally excluded." So, basically, this makes me wonder if maybe the problem isn't that bigger clothes on bigger women look unattractive, but that you're putting the wrong type of clothes on these bigger women. The standards that were first established were skewed and not representative of the population. Since then, these standards have been geared towards a narrow segment of the population. Of course, if you're designing something for an hourglass figure, it would look good only on somebody with that figure and not on somebody with a spoon shape. However, the inverse could also be true: if we designed something for a spoon figure, it might not look attractive on somebody with a hourglass figure. If you think about it, it seems that bigger women were historically considered more beautiful. Look at the paintings of Reuben. Its only been in the last 100 years that cultural forces have told us that thin was better.