Hi there,
Thank you for getting in touch with us, we do appreciate your time.
The zoom level that is set once a marker cluster is clicked is calculated based on where the markers in the cluster are, thus it would fit the map bounds to the markers within the cluster.
Thus unfortunately it is currently not possible to change the zooming level as it is automatically calculated based on each different clusters size and where those markers in the cluster are located.
I hope this answers your question?
Kindest Regards,
Matthew