Complex sentence: Nieuport developed an improved design specifically intended as a fighter-the Nieuport 11, which was regarded as the "baby" of the 10, which it closely resembled, except in size.
Split into simpler sentences: Nieuport developed an improved design specifically intended as a fighter-the Nieuport 11. The 11 was regarded as the "baby" of the 10, which it closely resembled, except in size.