21s do break the vast majority of birds that we see week in week out (45 yard max normally) and are super soft to shoot.
They are not tighter than 28s in my experience, just less shot. Yes they will break pretty much anything out there, and consistently too. But overall 28s will break a couple more at the top end of the field.
Difficult to say at the lower end as has been mentioned, the swing in scores would make analysis very hard.
As an interesting aside I shot a round of OT when getting the gun in Germany. Used a mix of 22, 24 and 28g 7s in STEEL (shock horror). The one bird I missed was with a 28. Not scientific but interesting. Vaguely...