Mr. Jay:
First, there isn't much difference in velocity for moderate upland field loads in most any gauge. Thus, the tight chokes are there for another reason in the 20 and 28.
That reason is pattern density.
Because the smaller gauges carry less shot for their bore size (the ridiculous 3" 20-bore excepted) tighter chokes help to keep more pellets within the 30" circle at most game ranges. This reduces wounded birds. If you miss, chances are you'll miss clean. If you hit, it may well be an effective and humane kill. (pardon the obvious oxymoron)
That's not to say an IC choke is ineffective. If you shoot quickly and are spot on, they work just fine. But you must be a bit more skilled.
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