Hi! I'm an obsessed teen so PREPARE FOR MANY WORDS!
1) It's about Calories In vs Calories Out. If you eat a meal worth 4,000kcal then YOU WILL GAIN WEIGHT! Chances are you need about 2,000 calories daily. You get slim in the kitchen but fit in the gym. Naturally skinny people just don't eat enough food to put on weight. Some barely even exercise and just eat Macdonalds [or whatever the stereotype is] but since they're only eating 2,000kcal they're slim [but not healthy[.
However, if you eat one meal worth 1,900 then you will probably lose (assuming you're overweight meaning that you're eating over 2,500kcal w/o adequate exercise)
2) Btw, please don't eat unhealthily. Junk foods are biochemically designed to addict us because we've evolved to crave fatty. salty and sugary foods [since in hunter-gatherer times, calories and sodium (chloride) were scarce and sugar/fat indicated high-calorie foods. This is why so many of us are fat [and even many "naturally skinny" people are only just eating junk food within their caloric requirements rather than actually eating healthily]. We're all a little addicted to that junk!
If you're going to eat unhealthily then only eat a small amount [like a 100-200kcal snack daily] and focus on healthy sources of carbs [potato, brown/wholemeal grains], fat [oily fish, nuts/seeds eggs], protein [beans, lean meat, and fish] and fruits and veggies.
I don't like many veggies so I try to eat the ones I DO like. I hate carrots, pak choi tomato [i know it;s a fruit but I find it in the veg section chill y'all] and parsnips so I try to eat pumpkin, spinach, sweetcorn, and cauliflower instead.
3) Also, try not to eat too little [especially if you're female. You don;t wanna stop your periods and risk osteoporosis]. Aim for 1,800 to 2,200 kcal daily [or 12,600-15,400 weekly] rather than depriving yourself with only 1,500kcal a day. You'll only freak out your body and put it on Starvation Mode [think "AHHH WE'RE GONNA DIE! SLOW METABOLISM AND LOWER IMMUNE SYSTEM COMPETENCY! AHHHHHHHH!"]
4) Lastly, PLEASE DON'T FOLLOW THOSE INSTA MODELS! They often promote expensive foods and smoothies or unusual diets/lifestyles while claiming "detox" and "cleansing" properties when in reality, you are already detoxing so long as you have AT LEAST ONE kidney, a pair of lungs and a colon. These models are only selling their GOOD GENES as weird diuretic/laxative-laden products or exotic fruits as if THOSE ware ere why their skin is so perfect and why their torsos are so long and curvy. If your blood really needed detoxing, you'd be in the hospital with septicemia or be on dialysis.
I hope this helped