The bill is built as a sandwich. The outer layer is keratin, arranged in overlapping tile-like scales that are themselves cemented together. Inside is a rigid closed-cell foam of thin bony trabeculae separated by air-filled cavities, with membranes between. The combination gives high stiffness for very little weight, and engineers have studied it as a model for impact-absorbing structures.
The consequence is that a bill occupying a striking fraction of the bird's total length contributes only a small share of body mass. That matters, because a solid bill of the same size would make level flight and perching on slender fruiting branches far harder. Toucans already fly in a laboured, undulating fashion over short distances.
The bill is not delicate, but it is not a weapon either. Toucans take eggs and nestlings and handle hard fruit, yet the structure is optimised for reach and light weight rather than for force.