The average human head has a circumference of 21-23", but if you were to knit a hat with that circumference, you would likely not have much use for the resulting garment. This illustrates the role ease. That is, knitting a garment with more or less fabric than the target form.
If a garment has negative ease, like a hat, it is designed smaller to stretch to size when worn. Alternatively, garments with positive ease have extra fabric worked into the design. These garments, like tops, have a greater drape and flow, thereby sitting away from the body when worn. When choosing a size, be sure to consider the amount of ease worked into the design.