This basically takes a Greek Avgolemono soup and turns it into a sauce draped over chicken with some spinach tossed in. I couldn't love this dish more.
I think the orzo ratio to the sauce is perfect as is (remember – the chicken and sauce are the stars of the dish, not the orzo) But if you want more orzo, add up to 1 cup total. Just make sure you add 1 3/4 cups broth per 1/3 cup of orzo (so 3 1/2 cups of broth for 2/3 cups orzo and 5 1/4 cups broth for 1 cup of orzo). And that also means to add the juice of another lemon + 1 more egg per additional 1/3 cup of orzo + 1 3/4 cup broth to balance the consistency and flavors out. If adding Parmesan and/or spinach, up that accordingly as well (or to taste).