The placket on your shirt should lay flat and not pull apart. Pleats on your shirt and pants and the vents on the rear of your suit coat should all lie flat and not be spread open when you are standing still. They are there so fabrics that don't stretch can allow for movement; they are not there to provide for extra room. On a suit coat, fasten the top button or top two buttons if it’s a 3-button coat, and always leave the bottom button unbuttoned, but make sure it hangs under the buttonhole and isn't spread apart. If anything is too tight or if the pleats and vents are spread open, try the next size up or a different cut that is better suited to your shape...not three sizes up. Your clothes should gently hug your shape and in all other respects conform to conventions of men’s fashion. Often, when guys are trying on coats, they will talk themselves out of a good fit because they do the Superman Pose and discover with their arms outstretched the clothes pull or become restrictive, and keep moving up a size until they find free range of motion in whatever size coat/paint tarp they end up in. Any good tailor or clothing attendant should refuse to sell you an ill-fitting coat, but oftentimes, the staff isn’t trained or you guys are buying stuff alone and doing the clothing-equivalent of not asking for directions. Let’s be honest, almost none of you fat guys needs to be able to do calisthenics or climb monkey bars when you are dressed nicely, so cut the this-feels-too-tight-when-I-raise-my-arms-up-like-this routine and know if you are dressed well, you'll probably be standing with your arms by your side or sitting down to type. Make sure your clothes fit well doing those activities.
Every single fat guy on the planet has worn clothes that are too big out of shame. We love clothing articles like sports jerseys or Jedi robes that are “supposed” to be baggy; we feel normal in them. It’s nice, it feels great; like hiding in plain sight, except it looks terrible. Many labor under the delusion that baggy clothes make us look smaller than we appear. Let me be frank: it is a lie. They make you look bigger than you are. The billowing extra fabric does not hide your girth, it accentuates it. Skinny people wear clothes that fit and unconsciously extrapolate that if you are wearing a big loose shirt, it fits you properly, so you must be that much bigger underneath. I also think most fat guys spent their childhood constantly outgrowing clothes and felt shame and disappointment when old clothes were too tight, usually after family buffet night. They probably had larger-than-necessary clothes purchased by mom to “last longer,” setting the stage for buying too-large clothes for a lifetime. It’s tough to break this habit when you are an adult and have settled into a more consistent weight. It’s true tight clothes make you look terrible in addition to feeling constricting, but the solution isn’t to go so far in the other direction you end up wearing a tent. Generally avoid clothes that add bulk or create artificial shapes like pleat-front, parachute, carrot-fit or wide-leg pants, cargo shorts, bulky shoes, and overly large or puffy sweatshirts, jackets, or hoodies. Also, if you have lost some weight, don’t continue wearing your out-sized clothes; any “bigness” in clothes not supported by your body hangs down as length which distorts your proportions, and creates bagginess, particularly under your arms. Alter or replace to look your best.