Switching APIs from 4.1 mini to 5.1 mini - a lot more tokens generated

We use 4.1 mini for our APIs and have many custom instructions to avoid answer specific questions, etc. When experimented with 5.1 mini the same instructions have generated a lot more tokens. We really cannot drop the instructions, they are important. Does anyone has advise on how to have comparable number of tokens used with 5.1 and preserve lengthy instructions?