軽さ is read as かるさ (karusa) and means lightness.
かるさ
karusa
かるい
indiscriminate
かるはずみ
thoughtless
けいかい
light (of movements)
かるがる
lightly
かるやか
light
けいはく
frivolous
けいじゅう
(relative) importance
かるがるしい
rash
けいこうぎょう
light industry
けいしょく
light meal
けいおんがく
light music
けいじどうしゃ
light motor vehicle (up to 660cc and 64bhp)