Complex sentence: The Italian solo cantata tended, when on a large scale, to become indistinguishable from a scene in an opera, in the same way the church cantata, solo or choral, is indistinguishable from a small oratorio or portion of an oratorio.
Split into simpler sentences: The Italian solo cantata naturally tended, when on a large scale, to become indistinguishable from a scene in an opera. In the same way the church cantata, solo or choral, is indistinguishable from a small oratorio or portion of an oratorio.