Actually the person found guilty after a not guilty plea is not getting a harsher sentence, they are getting the correct sentence according to the sentencing guidelines.
The person who pleads guilty gets a lesser sentence under the heading of 'contrition' but in fact the lighter sentence is not based on a moral premise but an economic one. The lighter sentence is indeed in exchange for not making the state, victims, police and witnesses suffer the stress, time and the cost of a trial.
It's the oldest quid pro quo going. You make it easier on us, we make it easier on you.
You don't make it easier on us, you get the standard sentence....