Judging by the size of the service, you are working of a EE stamped design. You would need to bounce your question off the EE. The NEC allows smaller neutral but you are dealing with some serious loads that could be harmonic in nature. You are better off by going with the drawing or get the EE to sign off on the use of a smaller neutral. If you were to "shoot from the hip" by installing a smaller the called for neutral, and something were to happen because of it, you, not the EE will be held accountable. The size of service tells me not to take the risk especially if it is to save few dollars on copper. Let the EE take the risk. Engineers are not totally useless.
A few projects ago, we had a contrator used a smaller neutral siting the NEC up one side and down the other and saying that we do this all the time and no one has called him on it before. I told him what I telling you and told them my office would accept it if the EE would sign off of it. They went through the motions and the EE stated in thoery the neutral could be smaller but it was not worth the added expense to crunch the additional info plus it could hinder future needs so he always goes with a 100% neutral so he would not sign off. The contractor had to re-pull the service wires at their expense.
