Complex sentence: Prior to recent results from the Kepler spacecraft most confirmed planets were gas giants comparable in size to Jupiter or larger as they are more easily detected however the Kepler planets are mostly between the size of Neptune and the size of Earth.
Split into simpler sentences: Prior to recent results from the Kepler spacecraft most confirmed planets were gas giants comparable in size to Jupiter or larger because they are most easily detected. However, the planets detected by Kepler are mostly between the size of Neptune and the size of Earth.