My current system has a 40L tank. Don’t I need a larger reservoir?

At first glance having a large reservoir seems beneficial as the fluid volume acts as a ballast to insulate temperature fluctuations. From an application perspective the volume of the tank in the circulator does not matter as the cooling or heating capacity required dictate performance. Think of it this way – two identical trucks need to go over a hill; one has a large trailer. The truck with the trailer will take longer to go over the hill. Likewise, a circulator with a smaller internal tank will achieve the set temperature faster than one with a large tank volume and responds faster to different set points or external events.