A good way to balance the durability of a unit is to compare its ability to absorb on a point-by-point basis. This is called resilience-per-point.
In other words, if 20 bolter shots will kill 1 plague marine, but kill 6.6 orks. A total of 24 points of PM will die, yet 39.6 points of orks will die from the same firepower. This means the resilience per point of the plague marines are actually much higher than orks when taking bolter fire.