The reason is because lifting power scales in proportion to the cross-sectional area of the muscle -- while mass scales with the volume. This is the complicated explanation. Imagine a human which could lift his/her own weight were shrunk down to 1/100th of his/her original size. Accordingly, they should be able to lift 1000 times his/her own weight. In terms of muscle power, humans are roughly 20 times stronger than ants. If ants were enlarged to our size, they probably wouldn't be able to lift themselves off the ground. --Mr. Ant