That Xtians & Jews had higher growth rates isn’t much in doubt, classical authors mention it. Tacitus is explicit on Jewish growth being due to their pro natalism.
How much growth over 300 yrs can never be determined now, just guesswork & obv Xtians have an incentive to overplay their numbers to make it out like Rome becoming Xtian was some inevitability, so I take Stark’s estimates with a pinch of salt. You can’t just indefinitely project 7 kids per women because as the groups bigger it gets harder & harder to swim against constraints that begin to impose themselves, just like how Iranian mullahs encouraged a baby boom during war with Iraq but then implemented family planning when they realized they couldn’t provide jobs for the youth bulge.