Adiposity: Being overweight may increase the chance of pregnancy - while taking oral contraceptives:

Overweight women who have not yet completed childbearing should consider using a backup form of birth control while on the pill, such as condoms or other barrier methods.Overweight and obese women who take oral contraceptives are 60-70 percent more likely to get pregnant while on the birth-control pill than women of lower weight, according to research published in the January issue of Obstetrics & Gynecology.

The results in the story are only given in terms of relative risk, but it appears that the absolute risk is a relatively small one:

Among 100 women taking oral contraceptives for a year, Holt's study suggests that an additional two to four women will get pregnant due to being overweight or obese.

It's a good idea to try to maintain a healthy weight, but don't assume that because you're overweight your birth control pill is useless. Far from it.