If you're overweight and do not want to conceive, be careful about the birth control options you choose. Let's talk about which ones are safest and most effective for you.
Carrying some extra weight can affect your hormones levels because estrogen and progesterone are fat-soluble. They get stored in your fat, leading to higher circulating levels of the hormones. Those higher levels may lead to estrogen receptor insensitivity. Since oral contraceptives work by suppressing the normal feedback loop by putting more estrogen in your body and tricking your body into thinking it's pregnant, higher estrogen levels produced naturally that your body is used to render the oral contraceptives ineffective. That may explain the higher pregnancy rate for obese women on oral contraceptives.
It also explains why heavier women have less luck in conceiving in assisted reproduction, like medicated IUI and IVF. Their bodies are insensitive to the medications that stimulate estrogen production, and they just don't get the response from the medications that a normal weight woman would. So if you're thinking of going through IVF and you're overweight, there is a good reason why your doctor will emphasize weight loss--your odds of success (and the doctor's) are much higher if you lose weight first.
But back to contraception. Oral contraceptives are not ruled-out completely if you're overweight, but they may be less effective, and the risk of a blood clot is higher. An alternative is the progestin-only contraceptive. It has more spotting associated with it, but may be a better choice.
Other options, of course, are the diaphragm and condoms. A diaphragm must be fitted to you by a doctor or nurse, and the size you need may change if you lose weight, so keep that in mind! Condoms are always an option and have other advantages, such as protection from most diseases (remember that HPV and herpes can be transmitted by skin-to-skin contact).
If you're in the process of losing weight, check with your doctor about your contraceptive options. If you have had irregular periods, you may find that your periods come into a regular cycle as you lose weight, and you may be at risk for pregnancy.