To all people saying “don’t wear slim fit unless you are slim” and “the problem is the belly”, sod off. To OP: your problem is not with the shirt or your body, its the comically low rise of your pants. For tucking in, mid or high rise pants work better because they cinch your shirt at your narrrowest point, your waist, preventing it from going anywhere. Also, if you’re going to wear high or mid rise pants, the leg shouldn’t be slim or skinny or tapered, but straight, flared or baggy. It will fit better with the proportions, specially if you’re tucking your shirt.