The answer is no. Your boiler capacity is determined by the heat loss calculation of your building. Although Low-H2O elements use less water and are smaller in size, they still provide the same amount of heat as conventional radiators. When the boiler also provides the domestic hot water, you have to dimension the boiler according to this. Never over dimension a boiler to much: because modern boilers modulate their capacity. A higher capacity means a higher output at the minimal modulation. The lower the output at minimal modulation, the more efficient the boiler capacity can be controlled.