Funnily enough my DD and I were discussing the 'larger size vs maternity clothes' issue today. She said, and I totally agree, that she cannot understand why people buy larger sizes instead of properly designed maternity wear.
She hasn't gained any weight anywhere apart from her bump, which is now rather large as she is nearly 8 months. At this advanced stage of pregnancy, 'larger' clothes would need to be at least two whole sizes larger than normal to accommodate her bump - and then they would be far too big across the shoulders and around the bust! When it comes to dresses and skirts, the bump would raise the hemline considerably at the front, giving an uneven appearance.
What is the point in buying clothes that are so much larger than you would normally wear? Post pregnancy they would be just as redundant as maternity clothes, so surely it's more sensible to buy things that actually fit properly and are comfortable to wear. Plus, it is far easier to sell used maternity clothes online than regular fitting clothes.