Complex sentence: During thelarche, the developing breasts sometimes are of unequal size, and usually the left breast is slightly larger; said condition of asymmetry is transitory and statistically normal to female physical and sexual development.
Split into simpler sentences: During thelarche, the developing breasts are sometimes of unequal size, and usually the left breast is slightly larger. This condition of asymmetry is transitory and statistically normal in female physical and sexual development.