One other factor to remember when designing a system is gravity flow. Most modern installations employ a flow check to prevent the water from taking it's natural path as it's heated and thus causing the boiler to fire. We have customers who have saved the expense of changing their plumbing over to primary secondary by simply allowing gravity flow and thus employing the capacity of the loop (s) as a storage tank.
Of course as has already been stated the design of the system, physical conditions and other factors need to be considered and taken into account by a qualified HVAC professional before this method is employed, or at the very least there needs to be a slight tolerance for a science project atmosphere while certain details are worked out.
One other thing to remember when the full loop has cooled to say 125* and is going to return to the boiler, if a boiler has more capacity it now has that extra capacity to heat as well when the big amount of cool water returns and dilutes the boiler. With only 12 gallons and most of it in contact with the fire, a LL WL 110 can recover faster than higher volume boilers designed using the tried and true method of mass storage.
One other thought, we tested the boiler in the UL lab using a total loss heat exchanger with 50 degree well water entering one side. The delta T was 110* and yet the boiler could still crank out 190* water at certain flow rates, so to limit flow to the loop with the large capacity can also control the availability of high boiler temp to ensure plenty of DHW production.
Then again a person can always do things the way they have always been done and get predictable results while never stretching the envelope. The mad professor in me always wants to see what can be accomplished.
I do not post this to convince anyone of anything, only to provide food for thought.