Complex sentence: Adding lower notes is somewhat impractical; as the bars become bigger and the resonators become longer, the instrument must be taller and the mallets must be heavier in order to produce a tone rather than just a percussive attack.
Split into simpler sentences: Adding lower notes is somewhat impractical because as the bars become bigger and the resonators become longer, two issues become important. First, the longer resonators present a difficult design problem involving the necessity of constructing a taller instrument and second, very heavy mallets become necessary to ensure the bars produce a tone rather than just a percussive attack.