The shoulders of your suit, for example, may end exactly at the edge of your actual shoulders, continuing in one smooth line down the rest of the sleeve. They may, alternatively, extend an extra half an inch to an inch. The line of the shoulders may be square and straight; they may be concave, curving down from the collar and then rising toward the outer edge; they may even be slightly convex.
Any of these styles may fit perfectly. If the shoulders are to extend slightly beyond your actual shoulders, and have a square, boxy line, they will require extra padding and support. If they are to curve naturally and with a slightly concave line, they will need to be carefully aligned with the line of your own shoulders, lest these ruin that line.
The point is, these variations create a different silhouette. They do not necessarily fit better or worse than the alternatives. Silhouette is more akin to colour or pattern – it is a personal choice, but one that can still be made badly (or, to be more generous, unsuitably).
The relationship between these two continues around the rest of the suit. The waist, for example, may be designed to be more or less pinched, creating a more or less defined skirt. If the suit is designed to have a generous waist, but you buy a smaller size to try and achieve a pinched waist, the wool will ripple with complaint when you button up the jacket. You have confused fit and silhouette – in trying to achieve the latter, you have failed in the former.
It is also likely to fit worse elsewhere, as you are deliberately buying a size too small. Your shoulders will press against the sleevehead. The back will feel constricting.
If the suit were designed to have a pinched waist, the wool would be darted, with slivers of material taken out and sewn back up again. The shoulders and back would fit fine and you would have achieved your desired look.
Silhouette is about what a suit is designed to look like. Fit is about whether a particular size of that design fits to your body. Don’t confuse the two. Be aware of what the suit and its designer are trying to do. Then judge its fit.
– As an addendum, a few quotes from Nicholas Antongiavanni about silhouettes: “Designer suits may be gargantuan or minimalist. With these it is not so much their level of comfort that fails you but their lack of harmony. A jacket that fits perfectly but is ridiculous in silhouette is useless, even more so than a jacket tasteful and sophisticated in silhouette that does not fit; for in the latter case in may be altered whereas the former is always harmful.
“When he said to me that the Americans do not understand fit, I replied to him that the French do not understand the silhouette, because if they understood they would not wear such square-shouldered, box-hipped, skin-tight jackets. The greatness of the English and the Italians as dressers is caused by their silhouettes, and France’s ruin caused by theirs. And because of Americans’ obsession with fashion, many of these have spread to our shores.”