The cross sectional area of the water flowing through the head determines the speed, volume, and pressure that enters the head. The more and larger the holes, the more volume you got, but at a lower pressure.
The smaller and narrower the holes in the head, the lesser the volume, but the better the flow rate of the pressure of the shower nozzle.
According to this theory, A 1/2 inch line without a head provides the most volume. Whereas, The 3/4″ valve was chosen because it provides more volume, letting it maintain more heads rather than larger ones.
The flow channel of a 3/4″ valve is less than that of a 1/2″ copper valve. So If you want more shower heads, you can go for both ¾” valve for hot lines and ½” for cold lines.