Antonyms for the phrase "carry heavy load" would involve lightness and ease. Some suitable antonyms are briskness, quickness, agility, and buoyancy. These terms suggest the ability to move quickly and easily without any hindrance. Other antonyms that can apply to carrying a heavy load include simplicity, effortlessness, and weightlessness. These words suggest an absence of difficulty or burden. Additionally, antonyms such as emptiness, featherweight, and sparseness imply a lack of a heavy load that needs to be carried in the first place. Overall, antonyms for the phrase "carry heavy load" describe lightness, ease, and simplicity rather than heaviness and difficulty.