Main and branches pipe size question

My old water heating system is a one zone reverse return system. There is no branch. All pipes are 1" except these sections connect to the CI baseboards. They are 1/2". Now I am adding 2 more zones. I have bought all pipes and fittings which are 1". I just realized that I am going to have branches now. And all branches have the same pipe size as the main. It sounds no good. My question is what will happen if branches have the same pipe size as the main? It will still work? Partially work?