Mcdonald's - Jena

I visited this morning and used the drive-thru. I mentioned that I had a voucher for a complimentary breakfast item and ordered the McSkillet meal without sauce. I also wanted to upgrade my drink to a larger size and offered to pay the difference. However, when I reached the payment window, the manager informed me that she couldn’t allow me to get a larger drink. I reiterated that I was willing to cover the extra cost, but she still refused. I believe customers should be happy, and this policy seems quite unreasonable!