Why Neutral and Ground Wire are Thin As Compare to 3-Phase Line Wire? in 3-phase 4-wire systems line wires are big as compare to neutral wire.
as per law ,

so,area of neutral wire is low ,then resistance of neutral wire is high.
so, heat loss is more.
as per P=I^2R
so,my question is why  we use thin wire for neutral.
 A: Because the neutral wire takes almost no current.
For a traditional big-three-phase-electric-motor thing, all the current in a three-phase system should flow between the phases: the fourth wire is just there to strap the whole thing to earth.
In cases where the phases are being sent to different customers such as domestic supply then they will all share the neutral, but things are set up so that the draws from each phase should be about equal, so the current in the neutral will again be fairly small.
A: You have one of three phases in ech of the three live wires. The phase difference between each parse is 120$^\circ$.  The returns for all three phases are combined together to form the neutral wire.  If the loads at the consumer end are non-reactive then the "return" currents will tend to cancel each other out and if the current taken from each phase is the same then the return current is zero.  
So result is that the net current in the neutral is usually smaller than the currents in the live wires.
If the current is smaller then the resistance can be increased by using thinner wires without too much of a loss of energy.
