[aruanan]

Both groups had "more" shrinkage. More than who it doesn't say.

Let's look at the excerpt below and see what it tells us about who is relative to whom in hippocampal shrinkage:

...or conversely, those who had no religious affiliation, had more hippocampal shrinkage (or "atrophy") compared to people who identified themselves as Protestants, but not born-again.

More hippocampal shrinkage <----> less hippocampal shrinkage

"born again" had more shrinkage than "no religious affiliation" that had at least the same or less shrinkage than "born again," but more shrinkage than self-identified "Protestants"

[TigersEye]

...or conversely, those who had no religious affiliation, had more hippocampal shrinkage (or "atrophy") compared to people who identified themselves as Protestants, but not born-again.

That was a very tortured sentence. But it was written by a PHD so it's not strange that it's difficult to sort out the intended causal relationships.