If you are obese, are you more likely to have an obese baby due to your actual size or due to the stuff in your diet?

Diet. There is good evidence that excessive weight gain during pregnancy may lead to macrosomic, or large babies. So eat a balance healthy diet while pregnant.
Various factors. A baby that measures large can be due to a number of factors such as genetics, diabetes in the mother (particularly if not controlled well) or excessive weight gain in the mother. If a mother is obese, then she is at an increased risk for diabetes; if she is not diabetic and minimizes her weight gain under md supervision, her baby frequently is normal weight.