pexplumbing
I have some plumbing that has been done in 3/4" PEX and the 90-degree brass elbow fittings that connect the lengths of tubing fit inside the tubing, so I don't see how these elbow fittings could possibly be 3/4" if they are inside 3/4" tubing. The elbows themselves look more like 1/2" pipes to me than 3/4" pipes.
What is the story here? Are PEX fittings smaller diameter than the tubing???
You need to evaluate where the leak is really coming from. Use some paper towels to thoroughly to dry the whole fitting area. Then watch to see where water first appears. I suspect that you may find the leak coming from here:
..as opposed to the PEX joint. It appears that you did not use a Teflon sealing tape on the pipe threads.
Since you mentioned freezing in some comments, I recommend this:
This is a freeze-proof hose bib, complete with anti-siphon valve to prevent water running backwards from your yard into your supply plumbing should the pressure go off.
Best Answer
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.