FIRST there was economic inflation. Then there was grade inflation. Now comes cup inflation.

While clothes designers have whittled down dress sizes so that even a musclebound woman can wiggle into a size 2, brassiere makers are defying fashion convention by selling bras with bigger cup sizes. These days, many women  to their shock or glee  are finding that DD is becoming the new C.

At Wacoal America, one of the largest bra makers in the country, 36DD is on track to replace 36D as its most popular size in 2009. (Three years ago it was 36C.) Bare Necessities, an online lingerie retailer, said 34DD was its No. 10 best-selling size last year, up from No. 17 in 2002. And this fall Elle Macpherson Intimates will introduce its first collection for fuller-busted women, offering cup sizes from D to G.

“It’s the ascendancy of the DDs,” said Noah Wrubel, the chief executive of Bare Necessities. “The bras that women are wearing these days are not their mothers’ bras.”

Theories abound as to why women’s cups seem to be (forgive the pun) spilling over. Some ascribe the difference to a confluence of health and beauty trends that have accelerated over time. A 2007 study of adult women by the Centers for Disease Control and Prevention showed that the prevalence of obesity  someone having a body mass index of 30 or more  has increased 2.1 percent since 2004, to 35.3 percent. Breast augmentation, too, is still a popular procedure, despite the recession. And age and hormones play havoc with baby boomers’ bust sizes, as well. But most manufacturers and lingerie shop owners say the phenomenon is due to the fact that more women are being fitted correctly. They call it the Oprah effect. Since 2005, when Oprah Winfrey devoted a show to how to buy the right size bra, the number of women seeking bra fittings has soared.