Complex sentence: When an inductive ballast is used, this essential in order to provide the "compactness" required for replacing incandescent bulbs, because the size of the ballast is inversely proportional to the frequency.
Split into simpler sentences: When an inductive ballast is used, this is essential in order to provide the "compactness" required for replacing incandescent bulbs. The size of the ballast needed is inversely proportional to the frequency.