If you start as a female character, you will have average breasts, or are flat chested as a male, but there are a lot of possibilities to change that. Breast Reduction and Breast Enlargement are spells to directly affect your breast size. Change Appearance spell will have a chance to enlarge them, if using the feminine variant, or shrink them, if using the masculine or elvish variant. A different way to change your size are consumables. The Large Mushrooms will cause your breasts to grow.