Quit smoking and weight gain

It’s common to gain weight after you stop smoking.
Smoking acts as an appetite suppressant.
Nicotine slightly increases metabolism. (Nicotine increases the amount of calories your body uses at rest by about 7% to 15%)
Smell and taste improve after you quit smoking.
Substitute snacking for smoking. (Some quitters adopt behaviours)

https://www.mayoclinic.org/healthy-lifestyle/quit-smoking/expert-answers/quit-smoking/faq-20058312

Nicotine replacement aids.
Low-dose antidepressant bupropion also helps prevent weight gain, and may work even better when you use it with nicotine replacement.
Stay busy during “smoke breaks.”: keeps your hands, mouth busy, “I wanted to get out of the house, so I walked instead.”
Eat mindfully.

https://www.webmd.com/smoking-cessation/features/quit-without-weight-gain#1

Your lungs and heart will be stronger
Your skin will look younger
Your teeth will be whiter
You will have better breath
Your hair and clothes will smell better
You will have more money when you are not buying cigarettes
You will perform better in sports or other physical activities

https://medlineplus.gov/ency/patientinstructions/000811.htm

The average amount of weight that people gain after stopping smoking is about four to five kilograms over five years. Most of the weight gain occurs in the year after quitting, particularly in the first three months.

Don’t crash diet. If you eat too few kilojoules, the body will respond by lowering the metabolism and burning muscle tissue for fuel.

Find ways other than eating to cope with withdrawal cravings. Some people drink water, while others count to 100 – experiment until you find your own method.

https://www.betterhealth.vic.gov.au/health/healthyliving/smoking-and-weight