Insertion vs epoxide formation

How can it be explained that in the first reaction mentioned below, a 7-membered ring dominates when the leaving group is nitrogen and an epoxide forms when the leaving group is dimethyl sulfide?


This is quoted from "*The Art of Writing Reasonable Organic Reaction Mechanisms*", Second Edition by Robert B. Grossman, [DOI: 10.1007/b97257](https://doi.org/10.1007/b97257).



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> The $\ce{C}$ in diazomethane is nucleophilic. The product of attack of diazomethane on the carbonyl $\ce{C}$ has a leaving group a to the alkoxide, so either a 1,2 alkyl shift or direct nucleophilic displacement can occur. The insertion product happens to dominate with $\ce{H2\overset{-}{C}-\overset{+}{N}\_2}$, but with $\ce{H2\overset{-}{C}-\overset{+}{S}Me2}$ the epoxide dominates.
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> [![enter image description here](https://i.stack.imgur.com/9OazT.png)](https://i.stack.imgur.com/9OazT.png)
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