Does being overweight or having large breasts make it harder to breastfeed?

No. Overweight or large breast should be no problem that would prevent breast feeding its encouraged for everyone.
No. Breastfeeding should not be affected by large breasts. However, initially after delivery it may take some time for the newborn to adjust and latch onto the breast and it may take some time for the milk to start coming on a regular basis. Remember, breast milk is the best choice for you and your baby!