Methane composition

I have a problem of ch4 overestimation in full scale digester. For small digester volume everything is fine, but for large digester volume, the total pressure increase and partial pressure of methane take the largest share (70-85%) of the total pressure hence the methane composition. How I could overcome this problem? 
thanks

I am afraid, this is far too little information to be able to suggest something meaningful.
Did you check why does the methane partial pressure increase with respect to the situation with the smaller reactor? Is it only this variable that appears to be overestimated (I would be surprised), or is there a more generalised 'shift' towards such irrealistic conditions?

All I can suggest is that you take a closer look at the model and try and isolate (or at least narrow down) those that may trigger this behaviour.