No, it's not about potatoes, but about potato products:
Eating more potato chips and French fries is likely to lead to a bigger weight gain over the years than the weight change associated with eating more of other foods, new research indicates.
Apparently these are worst than cakes and sugary foods
Marion Nestle, New York University professor of nutrition and public health, expressed surprise that potato products were linked with more weight gain than desserts like cake, cookies and doughnuts, which contribute the most calories to the American diet, other research shows. She says she suspects people who eat potato chips and fries also tend to eat too much in general, making these foods markers for a diet leading to weight gain.
Personally, I'm not surprised. The difference is in the industrial vegetable oil. While many baked goods do contain some vegetable oil, I suspect fried potato products contain much more.
More science on the study at Gene Expression. As with all self-reported population studies, serious problems loom. I suspect "whole grain" eaters are just health-conscious individuals. Most people who think they are eating whole grains are eating processed foods masquerading as them. Same goes from yogurt, the food of weight-conscious women, which is often spiked with tons of sugar.
What boggles my mind is that schools still serve fries and baked goods. Pretty much anyone who knows anything about nutrition, from low-fat vegan gurus like Joel Fuhrman to moderates like Marion Nestle to low-carb gurus like Jeff Volek, could agree that fried foods and sugary foods are bad for you. Yet they are on the menu EVERY DAY in most public schools. We should stop arguing about meatless Mondays and local vegetables and still focusing on this sort of trash we are shoveling into our children's mouths. If we can't eliminate these foods being served to our most vulnerable populations, that's just sad.