Why can ants carry items much heavier than themselves?

The reason that ants can lift so much is due to a concept of physics known as scaling. In fact the muscles of ants are no stronger than human muscles, but it is their small size that provides their advantage. Ants can lift so much because of the ratio between their body size and body mass; their small size means they do not have a large body mass and the proportion of their mass that is muscle is very high.
As a result they are able to lift weights that are many times larger than their own body weight. In comparison elephants have massive size and the proportion of their mass that is muscle is quite low; although they are able to lift incredible weights they are unable to lift or carry their own body weight. Humans have utilised the laws of physics to our advantage and we use levers and hydraulics to enable us to lift as capably as ants.