Their PA series necks are thinner than the Taylor standard carve necks, closer to Taylor's slim carves. Their Retro series uses a slightly thicker PA neck that is around the same thickness as the standard carves.
If you are buying a Martin though, I would recommend the Retro series, because they have the same unplugged tones as their standard series, whereas the PA series are braced like their lower series and as a result do not have a good unplugged tone.