Definition. "How much tasks a system can do relative to the size/number of components"
This is more like efficiency. But perhaps many complex systems in the real world are efficient. That's basically the hidden definition of the word complexity in Ecology and Biology.
In a network, the number of nodes and edges is the size of the system. And the number of connection of a node with other nodes is proportional to how many tasks a node can do.
If all nodes are connected with all other nodes, each node can do a lot, but it's very redundant, as well as each node has to do so many different things, although the system's function may be the maximum (Complete network).
I think 'complex' system implies such a system that does challenging things (which requires abilities of multiple components) very efficiently (with a relatively small number of components and a small amount of effort of each component)
