That's because it grew over time, to 40DD

 

Dolly Parton’s Faith Was Hard to Measure

Jason G. Edwards, Christian Century

 

Meanwhile: 

 

... in 1994, she broke her silence and told Vogue magazine (January issue, page 144): "I'm 40-20-36. I used to never tell anybody my measurements, but the older I get the more proud I am."