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???