Why Nothing Fits Off The Rack (And What To Do About It)

The opposite problem is just as common. The shirt fits fine across the chest but everywhere else it just droops. The shoulders slide down your arms a little, the body hangs loose and the whole thing looks borrowed.
What you want to check first is where the shoulder seam sits. It should land right at the edge of your shoulder, not past it. If that part fits well, a tailor can take in the sides and tidy up the length for very little money and it will look completely different on you. If the shoulders are already off, that is the one thing that is genuinely difficult to fix, so it is the first thing worth checking before anything else.

If your trousers feel tight at the front and have creases across your thighs, the cut is probably too slim for your legs. If the fabric sags behind your knees or bunches up around the seat, there is too much room there. You might think going up or down a waist size will sort it, but that won't fix either of those things. What you actually need is a different cut entirely.

The shoulder seam is the same story as shirts.

It should sit right at the edge of your shoulder, not drooping down your arm and not pulled inward. If that is wrong, nothing else about the jacket will ever look right on you and it is not something a tailor can easily fix.

If the shoulders sit correctly, most other things can be sorted. Sleeves can be shortened or lengthened. The body can be taken in. The jacket length can be adjusted. But the shoulders have to be right first.

Your outfit is always read as a top half and a bottom half. Where those two halves meet, and how long each one is, changes how your height and shape comes across. Most men never think about this and then wonder why some outfits just don't look right even when each piece seems fine on its own.

You want the eye to travel up and down your body cleanly without getting stopped anywhere. When trouser hems sit at the right point, when jacket lengths are where they should be and when your waistline is visible rather than buried under layers, your outfits will look more put together without you changing anything else about them.

You do not need slim fit everything. You just need clothes that follow your shape without clinging to it. There is a middle ground between skin tight and shapeless and that is where most men look their best.
Darker colours through the midsection help. Avoiding very bulky layers on top of each other helps. Keeping the fit cleaner through the shoulders and chest makes a bigger difference than most men expect, even if the rest of the outfit is fairly relaxed.