Nuts are great foods, one of the healthiest and tastiest and most convenient there is. I could eat them all day. But they are also a trap for the unwary because they are calorie dense due to the high oil content and so easy to consume. If you want to put on weight, start eating nuts.
I limit myself to some almond and hazelnut meal with my breakfast oats/barley, and the occasional packet of cashews/peanuts/macadamias/pistachios.
Incidentally, being rich in oils nuts are also a good treatment for constipation.