You have 1000 metres in a kilometre, but in a kilometre squared, you have 1000 metres squared (1000m x 1000m = 1,000,000m^2), or 1,000,000 m^2.
In 100km, you have 100 x 1000 metres, which is 100,000 metres. In 100km squared, you have 100,000 metres squared, which is 100,000m x 100,000m = 10,000,000,000 m^2.
In 100km, you have 100 x 1000 metres, which is 100,000 metres. In 100km squared, you have 100,000 metres squared, which is 100,000m x 100,000m = 10,000,000,000 m^2.