It probably seems like an odd idea, but it's true. You can't gain more weight than the weight of the food you eat. If you eat a quarter pounder of beef, the most you could possibly gain from it is 1/4 pound.
Now, of course, the world of healthy eating is far more complex than just saying "eat light things". You could easily get all sorts of nasty nutritional deficiencies like scurvy if you planned out your meals for the week based on how heavy the items are! But it's a very interesting thing to keep in mind. All other things aside - calories, carbohydrates, fibers, fats - the maximum total you could gain in body weight by eating something is the actual weight of the thing you're eating. It's simple physics. Your body cannot create matter.
So starting from that maximum value, you then account for things your body doesn't actually use - like fiber. Fiber goes in one end and out the other. So even if you ate five pounds of fiber in a yummy shredded blend, it would come right out again, cleansing your digestive system along the way like a scrubby roto-rooter. Weight gain - zero.
Water is another great, healthy thing to drink a lot of. It might seem backwards, but the more water you drink, the more your body is willing to let it flush through your system. If you only drink a little water, your body thinks there's a drought on and clings to every drop it can, causing water bloat. If instead you drink a lot, your body knows it is in a world with ample water, and lets the water go through you, carrying vitamins, and right out again. So you maintain the healthiest water levels by drinking a lot of it.
A main factor in how you eat is that you feel hungry or full. This is all based on the physical size of your stomach, being full of "stuff". If you fill it full of light, fluffy things, it will feel full without having a lot of calories / carbs in there. This is one reason that salads are so great. If you start each lunch and dinner with a salad, it helps your stomach to feel full, and gives you great nutrition, without loading you down with excess carbs. You can't gain a lot of weight, either.
Compare this with eating an entire box of heavy, dense chocolates. You can fit a lot of those in your stomach - and each one is overloaded with sugar. A five pound box of chocolates will probably cause you to gain ... five pounds!
Another point - your stomach is stretchy. If you keep overfilling it, it will stretch to be larger, and you'll have to eat more to feel full. If you keep eating small, regular meals, so that it isn't filled up, it will naturally shrink down a bit to better handle what it is getting. You don't have to be hungry!! Just eat small, regular amounts so you are satiated without being stuffed.
Remember, your body is an ever changing, adapting living thing. Be wise about what you put into it, and listen to the messages it is telling you. Eating healthy should never be about starving yourself or feeling hungry / thirsty. If your body thinks it is in a famine situation, it will cling to your fat in desperation. You need to eat healthily, so your body feels safe, secure and happy. Only then will it release its fat as unnecessary, excess stuff.
Lisa Shea's Library of Low Carb Books