(They'll likely get a bit bigger next year.)
The internet tells me they should be three feet apart, which they currently are, roughly, but that seems a bit bare and empty Maybe there's something else that should be planted between them?
"Bedding dahlias can be planted 9 to 12 inches apart. The smaller flowering types, which are usually about three feet tall, should be spaced two feet apart. The taller, larger-flowered dahlias should be spaced three feet apart."