The simple answer is there isn’t a standard “perfect fit”, but understanding the concepts of wearing ease and design ease can help you figure out what works for you.
Wearing ease is the room added to a garment so that the garment feels comfortable and allows for normal body movement such as raising and lowering the wearer’s arms. What feels comfortable to one person can feel too tight or too loose to another. Other factors such as whether the fabric is stiff or soft, whether the garment will worn alone or over other items, and what the wearer will be doing while wearing the garment also need to be considered as do cultural and societal norms. With that said, the commercial pattern industry generally states that minimum wearing ease is recommended to be 2 1/2″ (6.4cm) at the bust area, 1″ (2.5cm) at the waist and 3″ (7.6cm) at the hip area.
Design ease is any extra space that is purposely added to a garment by the designer to achieve a certain look, or drape. How much or how little determines whether a garment will be close-fitting(less than the minimum wearing ease), fitted (minimum wearing ease), semi-fitted or fluid (slightly more than the minimum ease required for normal movement), loose-fitting (width is greater than minimum requirements for wearing ease) or very loose-fitting (little or no shaping over body).
The above guidelines are for woven fabrics because knit and stretch fabrics have different properties. With knit and elastic fabrics, the garment’s finished measurement is often equal the wearer’s body measurements because of the fabric’s ability to move with the body. The knit’s structure, stability, and its percentage of stretch will provide a guideline for the ideal amount of ease to add. Some fashion knit tops on the market today purposely have an excess of positive ease to create an attractive drape, movement, and comfort. On the other hand, garments made out of very stretchy fabrics, such as spandex leotards, may actually be made smaller than the body. Negative ease is often found in garments such as active wear and swimsuits.
OK, on to your sweater question. To figure out your”perfect” fit, you need to start with your actual bust measurement and then figure out the look you’d like for that style of the sweater. In most cases, you’ll be looking for a sweater 5-10 inches larger than your bust measurement. The smaller your bust, the less ease you'd need proportionally than larger bust, i.e. a 32" bust would be fine closer to 5” extra, while a 42" bust would want closer to 9 or 10". Most pictures style the models with a great deal of positive ease, which results in a slouchy/boho look. Many women, though, prefer a fit much closer to their actual measurements with very little positive ease. Both options are equally "right," depending on what you want the final look to be. The weight and density of the knit can also influence your decision. A fine, or loose, knit will stretch and drape significantly more once it’s on the body than a heavier, bulkier knit. Again, the “perfect” fit depends on what look you prefer.
