Original: Although the original Borough covered a large area it included considerable areas that were unsuitable for development and already the population was expanding into the surrounding districts.
Simplified: Although the original Borough covered a large area it included big areas that were not good for development and already the population was expanding into the nearby districts.
Simplified: Although the original Borough covered a large area it included a lot of areas that were not right for building on and already the population was growing into the surrounding districts.
Simplified: Although the original Borough covered a large area it included major areas that were unsuitable for development and already the population was expanding into the nearby areas.