The dealer was probably saying, that in order to get the nominal BTU rating of the boiler (a very nebulous number anyways), you'll need the larger line. You can only pump so quickly through the pipe without cavitation at the pump, and eventually you can't make up for cross sectional area with flow speed. Does it make a difference in your setup? It doesn't sound like it makes a difference for you as long as it's heating your house properly all winter. Is it worth changing out at this point? It probably does not make sense to spend that kind of money to go larger on the pipe to be able to downsize your pump.
Now if you weren't getting enough heat, then upsizing the pipe is worthwhile because you can squeeze more BTU out of the system if you can physically move more water through.
That being said, I went with 1-1/4" on both installations so I can get as much heat as possible out of my system if I wanted to. I can always slow down my water speed or downsize the appliances (smaller heat exchangers), lower my water temp or narrow my delta T if I didn't need as much heat from it.