Thanks for your question, Nathalie!
It is true that ants are proportionately much stronger than we are. I don't think any human could dangle from the ceiling with 100 times his or her body weight, like the weaver ant Oecophylla pictured here. There are many adaptations that are working together to allow ants to perform impressive feats like these: hairs on their feet that can stick to very smooth surfaces, large muscles in their heads to close their jaws, and light lean bodies. Most worker ants don't have functional reproductive systems, so their strength-to-weight ratios are higher than many other insects that are weighed down with the burden of perpetuating their genes.
However, comparing the proportional strength of even the strongest humans to an ant is unfair. Even lions, tigers, and bears (oh my!) can't lift more than 10 times their own body weight, as many insects can. Some of the physics behind this is explained here. The strength of ants is "super," but it is not super-natural. It all makes sense once you know a little more about physics.
Briefly, smaller organisms will always have bigger strength-to-weight ratios, because it's the surface area of the muscle cross section that determines strength, but the volume of the animal that (all else being equal) determines mass.
Less briefly: imagine three perfect cubes, each of a different length: 2cm, 5cm, and 10cm. The 2cm cube has a cross-sectional area of 2x2=4cm^2 and a volume of 2x2x2=8cm^3. The 5cm cube has a cross-sectional area of 5x5=25, and volume of 5x5x5=125, and the 10cm cube has cross-sectional area of 10x10=100 and 10x10x10=1000. If these cubes were animals (admittedly, very strange ones), the 2cm cube could have a strength-to-weight ratio that was proportional to 4/8, while the 5cm animal's strength to weight ratio would be 25/125 = 1/5, and the largest, 10cm cube-animal would have a strength-to-weight ratio of 10/1000 = 1/100. Of course, this is an oversimplification, but I hope this helps clarify why all of the proportionately strongest animals are very small.
Although this is an ant blog, I feel it is only fair to point out that ants are not the strongest insect, even proportionately to their body weight. The prize goes to a dung beetle, which can drag more than 1000 times its body weight. These beetles are larger than any ant, which makes their strength-to-weight ratio even more impressive.
The feat of strength in which ants have beetles beat is how rapidly some of them can close their jaws. Ants of the genus Odontomachus can close their jaws at speeds of up to 230 km/hr (143mph), generating a force that is 500 times their body mass. Not only are these forces very effective at subduing prey and smaller enemies, some of them can use their jaws to launch themselves into the air. This youtube video is completely worth checking out if you like wathching slow-motion ants flail through the air. For a more dignified synopsis, one of the original articles on Odontomachus jaws is here.
I hope this helps!
Jesse Czekanski-Moir & the AntAsk Team