Whereas in elasticity and plasticity geometrically similar structures behave in the same way, when strain-softening and strain-localization are taken into account the structural behaviour ranges from ductile to brittle merely by increasing size and keeping material properties and geometrical shape unchanged. The size-scale of plain and reinforced concrete elements has often revealed a fundamental influence on the global structural behaviour. This has been theoretically predicted and experimentally confirmed by the results obtained so far. As a consequence this is likely to influence several aspects concerning the design of large structures. The paper emphasizes how the size effects are also present in R.C. elements with a low content of steel and outlines the implications on the minimum steel percentage for these structures.