Converting a recipe from 6qt Instant Pot to 8qt
The answer is you may have to experiment.  

I believe the problem lies in the fact that more water takes longer to reach temperature, given the same burner arrangement.  This means whatever you have inside the pot is exposed to the lower grade heat for a longer period and by the time you reach full pressure, the food may already be over cooked.  I have not tried this myself but consider using preheated water at the beginning in order to shorten the overall cook time.