As far as I can tell, part of the 4e mandate is actually to depict all races as "attractive."
There is a range of attractive -- dwarves are pretty plump, for instance. But the skew is away from the pudgy phisique to either a muscular one (the half-orc or the goliath) or a svelte one (...almost everything else...).
Part of the reason for this would be that, when depicting a race, WotC wants you to WANT to be one. Most modern people don't want to be pudgy, even if they are (or, as is often the case, even if they're not, and just think they are). It's not good when you're in the "fantasy" business, in the same way that being an outcast rebel badass is good when you're in the "fantasy" business.
Often in mythopoetics, the chubbiness comes from health, youth, wealth, and power. Often, also, with joviality and friendliness. For most of human history, the big gals and the big guys were more powerful, more wealthy, and more healthy than the malnourished commoners. Plump halflings in tolkein and plump gnomes in myth represented their generally benevolent nature -- they were pranksters and workers, friends who would share their wealth with you.
In today's society, as weight becomes associated with the stereotype of "lower class," uneducated, ill-mannered, messy, and unhealthy, thinness has become an icon of wealth, health, and power. Widely available, unhealthy food has meant the denial of that food is an elite position to hold -- it's more expensive to eat a salad than it is to eat a hot dog, so people who are thin and eat salads must have more power than those who are chubby and eat hot dogs.
It's a social dynamic.
In 4e art, WotC probably smartly mirrors that dynamic, because you're going to sell more books telling people what they want to hear rather than telling them what they don't.
But even there, it's not universal -- dwarves are still solidly built, goliaths are muscular instead of thin, half-orcs too. Races that have a reputation for hardiness and strength tend to be beefier (it's hard to tell with dragonborn sometimes, but few of them are really thin). No D&D race really has a reputation for joviality, friendliness, or fun-loving hilarity, which makes some sense: WotC wants you to see all the characters as dangerous fighters and risk-takers, not as Santa Claus.