Proportionally, most male body fat is deposited around the waist. The fact you have "a little" fat there is perfectly healthy, and you would probably really struggle to lose it.
However, don't purposefully eat unhealthily in order to gain fat. In fact, don't aim to gain fat at all. Gaining fat will NOT help you get bigger chest or arms.
Gaining fat will reduce the effectiveness of your training. You should get calories by eating carbohydrates, not fat, so that you burn them while training your arms and chest, and have a more intense and thus productive workout session.
Leftover calories will turn into fat, but you should try not to have leftover calories: aim for a balance, the amount of energy you eat should match the amount of energy you burn during your workout, and the recovery and growth of those muscles.
If your goal is to hypertrophy your chest and biceps, then eat 150-300 calories above your maintenance calories. If you want to pay attention to dietary specifics, then you can eat about 0.4g of fat per pound of bodyweight every day. The rest of your calories (by far the majority of your calories) should come from slow-release carbohydrates. Make sure you eat enough protein to see hypertrophy results (normally about 1g of protein per pound of bodyweight per day).
Strength athletes like powerlifters or strongmen are often so fat because fat gain is an unavoidable side effect of eating as much as they do (10,000 calories a day) to maintain their muscles. Bulking always inevitably comes with some fat gain, and these steroid-fuelled world elites are just at the extreme end of that. A high bodyweight provides a stable foundation while moving massive weight. They have built up to their size over years, don't copy them; suddenly switching to a 10,000 calorie diet is not the optimal way to increase the size of your muscles, and would be very unhealthy. Fat itself costs calories to maintain, calories you would rather spend on working out your muscles and growing your muscles, and having too much fat is unhealthy.