I am not sure why they are unusually high, but nothing really "happens" if you don't pay the full amount. The potential problem is the underpayment penalty if you haven't paid 100% of the prior year's tax or 90% of the current year.
If you think they are inflated figures, I would recommend using your best judgment. If you are off slightly on your calculation, there is no penalty if you owe less than $1,000 and the penalty is only 0.5% of tax owed per month late.
