A typical WH has maybe 30-40K BTU input (see what yours has). Your boiler probably has significantly more. Usually, when an indirect is installed, it becomes a priority zone so all of the heat from the boiler goes to it while it needs heat. So, a smaller tank will usually outperform a standalone WH. Look at the 1st hour delivery specs for what you have and that new indirect...the smaller indirect will probably have a greater quantity of water it can deliver in that first hour and will recover faster because it is more efficient and you have a greater source of heat to it.