The inside diameter (ID) of PEX fittings is smaller than the ID of the pipe, and the ID of the PEX pipe is smaller than the same size copper pipe. When the system was designed, the outside diameter was kept constant but the inside diameter shrank (compared to copper).
This sounds like PEX might have much lower flow rates, but the silver lining is that PEX is flexible so it has a lot fewer fittings. The change in direction in fittings of copper pipe is a flow bottle-neck even if the fittings are larger than the pipe ID. It's possible to have an entire run of PEX with no fittings at all until it comes out of the wall.
Now, even with that advantage PEX might have a lower flow rate, but it should still be much higher than any point of use calls for outside of a bathtub. Also, new PEX installs use a manifold system so you don't have 2-3 points of use coming off of one 1/2" line.