The "answer"/opin I have is the 21s will have a nearly identical rolling circumference to the 20s,
and thus be very close to the same overall "circle" within your wheel well; thus, the tires will be wider
and lower profile but that set may not necessarily "look bigger" in the wheel well or, on the car, especially to
those viewers not tuned in to BMW wheel styles.
My other opins are that 215s are a Titz looking wheel, but pricey,
but may not be the best application for rolling around the streets
of NYC.
GL, mD