OAL and weight matters.
A 21" 870 is 41.5" long and weighs about 7 lb.
A 26" Winchester Select Deluxe Field O/U is 43" long and weighs about 7 lb.
Pretty similar, and they'd have pretty similar handling in the field.
Remember that a receiver gun is about 3-4" longer than a double gun.
Long guns = natural smoothe swing, slower to the shoulder, get hung up in brush, good on almost all moving targets
Short guns = little or no inherent swing, quick pointing, easy carrying in brush, good only on rising targets
If you're shooting fast-rising birds flying away from you, in the brush, a shorter and lighter gun will probably net you more birds in the pot.
If you're shooting birds passing you going right or left, in relatively open space, a longer and heavier gun will probably net you more birds in the pot.
That doesn't mean you CAN'T hit a crosser with a short gun, just that you probably won't hit 'em as often.