Why can a tiny balsa bridge hold so much more than its own weight?

Short answer: two reasons: good structure, and small size. A truss puts material where the forces are, and small structures carry more relative to their own weight than large ones.

1. A truss uses material well

Every stick carries a push or a pull along its length, which is the most efficient way to use a material. Nothing is wasted holding up a flat slab. See how a truss works.

2. Small things are relatively strong

If you double every dimension of a stick:

• Its strength, which depends on cross-section, goes up by 4 times.
• Its weight, which depends on volume, goes up by 8 times.

So big structures are relatively weaker, because they must carry their own growing weight. A tiny balsa bridge has little weight of its own to carry, so most of its strength is available for the load.

What this means

• Reported balsa bridges holding 1,000 times their own weight or more are plausible at contest scale. See how much a bridge can hold.
• A real bridge many times larger cannot do the same, which is why real bridges are not built like models.
• It also explains why the efficiency score is so large for model bridges.

Treat the scaling as an idea, not a design rule, since glue joints and buckling matter too.