This also happened to the original Minis in the form of a new, larger car based on the Mini's platform concept. That concept was the work of Sir Alexander Arnold Constantine Issigonis, CBE (1906-1988), biographical information here.
The original Mini (produced 1959-1967), was known internally at British Motor Corporation as ADO15 -- Amalgamated Drawing Office project number 15. The Wikipedia entry is here. It was followed by ADO16, produced 1962-1974, and like the Mini was badge-engineered and sold by several BMC brands. More information can be found here.
An early ADO16 was the Morris 1100, subject of this post. These cars were larger, higher priced, and more "styled" than Minis. The Mini design gives the appearance of being more an engineering exercise than a styling one: only the grille looks like it passed under a stylist's pencil. The Wikipedia entry on the ADO16 mentions that the Pininfarina firm was involved in its styling.
Due to its small size and proportions, the Morris 1100 is not a beautiful car, and most likely never could be. What we see here are really two designs spliced together. One is the above-the-beltline greenhouse that seems more appropriate for a large, more standard size vehicle. The other is the lower body that is in synch with the size of the wheels, thereby creating a sensible composition for that half of the car.