A bit puzzled by this one. When I add new music to iTunes on my Mac, the songs are getting downloaded to my iPhone. What setting is allowing that? To clarify, not just iTunes Match available on the phone, I mean offline listening kind of downloaded.
Right now there is a bug that makes iOS devices randomly download music added to iTunes Match or Apple Music. There is a thread on it here. Not only is it annoying, but it's also a battery drainer and data hog. Hopefully they fix it soon.
In the meantime, check Settings->iTunes & App Stores on your iPhone and make sure that music is not checked for automatic downloads. If music isn't checked then songs shouldn't download once Apple fixes the bug. Hope this helps!