I know that resonances in particle physics are the creation of metastable bound states (i.e. particles). I also know that near such a resonance the decay width can be described by a Breit-Wigner curve. But I am confused about why we actually, physically get an increase in the cross section. Consider for example the reaction $e^{-}+e^{+}\rightarrow c \bar c\rightarrow q+\bar q$ and $e^{-}+e^{+}\rightarrow q+\bar q$ Feynman diagrams for which are shown below:
Naively I would expect second one to have the larger cross section due to the fewer number of interactions. But this is not the case, as can be seen from this pdg plot. My question is therefore:
Why do particle resonances cause an increase in the cross section of a reaction?
It must be pointed out that I am unsure if the reaction from $c$ to $d$ is via $\gamma$ or $Z$ (please let me know) but this should not change my argument.