My dog was 65 pounds at 16 months too. She was 45 pounds at 5 months.
She is about 68 pounds at 18 months. A lot of people do the math too early and think their dogs are going to be huge. Dog stop gaining rapid weight at about 6-7 months...it drops off quickly. I figured, wow, she's only 5 months and 45 pounds, she'll probably hit 85. I was wrong. She will probably never go over 70.