Does diet soda cause a bigger waist size?

This MSNBC article has this quote

In one study, people who drank two or more diet sodas a day had five times the increase in waist circumference over a 10-year period compared to people who didn't drink any diet soda. 

The research is also referenced here:

For one study, researchers at the center followed 474 diet soda drinkers, 65 to 74 years of age, for almost 10 years. They found that diet soda drinkers' waists grew 70 percent more than non-drinkers. Specifically, drinking two or more diet sodas a day busted belt sizes five times more than people who avoided the stuff entirely.

What about this study?

The other study may hold the answer. In it, researchers divided mice into two groups, one of which ate food laced with the popular sweetener aspartame. After three months, the mice eating aspartame-chow had higher blood sugar levels than the mice eating normal food. The authors said in a written statement their findings could "contribute to the associations observed between diet soda consumption and the risk of diabetes in humans."



What do you think of this? What other reasons could  cause this? Make sure you read through the entire article and identify the good and bad about the research. 

Update - New article from NBCnews.com