Oh, it is possible! We can just as easily consume too many calories from 'healthy foods' as we could from junk foods. Weight gain is not prejudice to the kind of calories you are eating. If you eat more calories than you burn throughout a day you will predispose yourself to gaining weight.
I was talking with a friend today that asked me if he should have both a sandwich and smoothie after his workout. Knowing his goals (to get lean and mean), I recommended that he eat 1/2 of the sandwich and drink only 1/2 of the smoothie. I wanted him to save the rest for later and to S P R E A D those calories out throughout his day. He ultimately decided to have one not both!
He then reminded me that, 'its not good to eat a bathtub full of any kind of food!'
Funny, but its true! Remember everything in moderation...here is a small list of some 'healthy' foods that people tend to overeat on and what needs to be conscious of.