Growth in the quick-serve, fast-casual space is very fickle. All the signs of a strong brand can be present, yet when a brand expands beyond it’s current turf, failure can be swift and fatal.
For the record, size doesn’t matter when it comes to success. There are small brands (under 500 locations) and larger brands (some with tens of thousands of locations) that have seem amazing and profitable growth. An each brand has a different approach to how it works with franchisees. Chick-fil-A as an example has a rule where operators my only have one location whereas others encourage multiple units.