That's why larger csa is needed for Bonding Conductors.
The real reason for large CSA in bonding conductors is that they are part of a circuit that has no fuse or MCB to limit fault current in that circuit.
Bonding the Main Earth Terminal (MET) of a house with PME ( Earth connected to Neutral ) to a low impedance ground such as a metallic supply pipe allows a portion of the Neutral load current to flow through the bonding top ground and hence to the Neutral of the substation on parallel to the Neutral conductor to the sub station..
When there is a lost network Neutral ( metal theft etc ) bonding conductors in houses may be called on to carry the full local area Neutral current to ground. In most cases the impedance to the ground will limit the current to a value that the bond conductor can safely carry. There have been several incidents reported in the media where a bond wire has over heated and caused damage to property.