Have him redo the plumbing using standard sized plumbing, or maybe theres some sort of reducer. What was his reason for using odd sized plumbing? I hope a pro gives an answer.

Is the shower outlet 3/4", or just the inlets?

Yup. More info needed. I'm of the same mind as catbuilder.

If the plumber ran 3/4" supply lines to the valve, installed a 3/4" valve, and then ran 3/4" tubing out of the supply valve to a 3/4" threaded dogear buried in the wall with the expectations that a standard 1/2" shower arm and 1/2" shower head would be installed, something is indeed curious.

Have you asked your plumber?

Depending on the shower fixtures, if a 3/4" valve was installed, exiting the supply valve is where the plumbing is typically taken down with a reducer from 3/4" to 1/2". A 3/4" run of tubing out of the valve would be great for feeding a loop of eight body jets. But for a TYPICAL single low flow shower head, 1/2" is fine.

With all that written? There are some shower heads that have a 3/4" threaded inlet. They are typically the higher flow rainshower/dousing heads, but they typically get plumbed to the ceiling of the shower.

So it depends on your shower head. While most are 1/2", there are some that are 3/4". Was a specific shower head spec'd out in the build proposal?

Talk to your plumber. Or your GC if you have one. Ask, see what they say. Could be a misunderstanding and they are best resolved sooner rather than later.

Good luck!