  Physical networks are made of nodes and links that are physical objects
embedded in a geometric space. Understanding how the mutual volume exclusion
between these elements affects the structure and function of physical networks
calls for a suitable generalization of network theory. Here, we introduce a
network-of-networks framework where we describe the shape of each extended
physical node as a network embedded in space and these networks are bound
together by physical links. Relying on this representation, we introduce a
minimal model of network growth and we show for a general class of physical
networks that volume exclusion induces heterogeneity in both node volume and
degree, with the two becoming correlated. These emergent properties strongly
affect the dynamics on physical networks: by calculating their Laplacian
spectrum as a function of the coupling strength between the nodes we show that
degree-volume correlations suppress the role of hubs as early spreaders in
diffusive dynamics. We apply the network-of-networks framework to describe
several real systems and find properties analog to the minimal model networks.
The prevalence of these properties points towards general growth mechanisms
that do not depend on the specifics of the systems.
