Are you more comfortable out on a sunny day with constant radiation, or when a cloud covers the sun? Having the boiler run constantly at the proper output means the radiators are a relatively constant temperature rather than getting super hot for a few minutes, cooling down, then getting hot again. An oversized boiler just can't run constantly to MATCH the heat lost, it MUST cycle. A right-sized boiler runs at the design temp, and with smart controls, can adjust its output to match the need...just enough heat in to match that lost - i.e., constant temp, constant comfort. Gross example...take a blow torch, momentarily point it at something, then turn it away (simulating turning off)...you can achieve a reasonable average temp, but it won't be particularly comfortable. Just like a dimmer on an electric light can get just the right illumination, something like a mod-con can achieve just the right heat input to get things 'just right'. This only can happen when the size is matched to the load.
Not having enough radiation capability is a problem, but you have plenty. The only time a big boiler might be required is if you used a huge setback on the coldest night of the year. With today's better boilers, a small setback is okay, but your comfort level and fuel use is likely to be similar by just setting and forgetting, or a modest reduction if you like to sleep cooler. Personally, I just adjust what covers I use and leave the thermostat the same. Before I finished my remodeling, I used a hydro-air system...the boiler was and is more than adequate, but the heat exchanger I had was not able to provide enough heat to allow a big setback, but worked fine when using a modest one or keeping things constant. In this case, I was radiation limited. Then, I added in-floor radiation, and that problem went away. Constant temp allows it to be comfortable, regardless of the outside conditions. ANd, with the smaller boiler and running more efficiently, my fuel bills went down significantly.