Complex sentence: Some of the GAU-8/A technology has been transferred into the smaller 25 mm GAU-12/U Equalizer developed for the AV-8B Harrier II aircraft, which is about the same size as the 20 mm M61.
Split into simpler sentences: Some of the GAU-8/A technology has been transferred into the smaller 25 mm GAU-12/U Equalizer developed for the AV-8B Harrier II aircraft. The GAU-12 is about the same size as the 20 mm M61.