The only reason you would want to reduce the size of the chain (and sprockets) would be to reduce weight.
A 104 link DID 525 V chain weighs 2033gms
A 104 link DID 520 VM chain weighs 1607gms
A saving of 426gms
The std OEM 15T front sprocket weighs 267gms
A AFAM light weight 15T front sprocket weighs 211gms
A saving of 56gms.
A AEM magnesium sprocket plate with a 44T aluminium sprocket titanium cush drives and an AEM drive hub weighs 2230 gms.
A 42T 525 OEM steel sprocket has got to weigh 3500gms (I dont have one handy to weigh)
So around 1.6 kilos can be saved.
Worth it? well you will not notice the differernce in spin up vs fitting a light flywheel, but 1.6kg of rotating weight reduction (allbeit the slowest speed in the drive line) is not to be sneezed at.
You will suffer increased wear on the sprockets due to the load being applied to a smaller area.