Why are smaller animals stronger than larger ones, when considered relative to their body weight?
Zasso pointed it already out:
Scaling up a ant to human size means volume (weight) increasing by length proportional $l^{3}$, but the force of muscles is determined by cross section (not muscle weight), so muscle force goes proportioal to $l^{2}$.
Smaller factors are likely:
- stiffness (or strentgh of the skeleton)
- balance point (center of mass)
- leverage (human skeleton is "sub-optimal" for this, we are afaik best optimized by evolution for long runs, more than any other animal)
i did some quick further search on "robot insects" on this interesting topic. This article is quite worth reading and relating biological to technological limits as well as current state of the art in nanobionics:
Interestingly, the force generated from a wide variety of actuator materials and devices has been found to be surprisingly invariant when compared with the actuator mass. A few years back, a comparison of the force-to-weight ratio of various organisms and machines found a striking similarity, with the force scaling linearly with mass over 20 orders of magnitude – from individual protein molecules to rocket engines ("Molecules, muscles, and machines: Universal performance characteristics of motors"). Remarkably, this finding indicates that most of the motors used by humans and animals for transportation have a common upper limit of mass-specific net force output that is independent of materials and mechanisms. Therefore any actuating device produces the same force per mass regardless of the material from which it is constructed and the mechanism by which it operates. This study also makes clear that biological systems dominate at the small mass, small force, range. In contrast, human-made machines dominate at the large mass range.
short example as Sonny asked for in comment:
ant with 10 mm length & 10 mg mass
$\Rightarrow$ lets scale up to human size (2m) $\Rightarrow$ means a factor of 200. So the mass scales with 200x200x200=8000000 (Volume $\propto$ $l^{3}$ ) $\Rightarrow$ human sized ant=80 kg. But muscle forces scales only by factor 200x200=40000. The small ant can carry 100x10mg of her own mass=1g, the human sized ant should be able to carry 1g x 40000=40 kg.
Conclusion: pretty comparable to a avg. 80 kg human man able to carry 40 kg!