Complex sentence: Heimaey was about 11.2 km² before the eruption but the "island" grew by about 2.24 km², so the island became about 13.44 km² when the eruption finally stopped.
Split into simpler sentences: Heimaey was about 11.2 km² before the eruption, but the "island" grew by about 2.24 km². The island measured about 13.44 km² when the eruption finally stopped.