The issue for me is getting a heavier bag onto and off of the train safely. There is the gap and then often several very steep steps. Also keep in mind that at some stations we have found no elevator and had to carry the bag up or down a flight of steps to get to the track. A larger bag can also be more awkward to handle in these situations even if it's light, especially if you're on the shorter side. Been there, done that with a large, heavy suitcase, and never again for us.
In the long run it depends on your own personal factors such as strength, age, fitness level, etc. No one else can tell you if it will work for you. I would advise packing your proposed suitcase with the approximate weight you plan to pack testing it out going up and down stairs. See not only if you can do it, but if you really want to do it for your entire trip.
One suggestion if you find the larger bag too awkward...we take carry on only, but also each pack a sturdy fabric bag folded up in our suitcase. We put the items we acquire along the way in that extra bag which attaches to the top of our spinner carry on bags. For us it's easier to handle the 2 smaller pieces.
Have a wonderful trip. P.S., we've also learned to buy small souvenirs!😁