Complex sentence: While Westfield Whitford City was been gradually upgraded over the years, the most extensive development occured between 2002 and 2004, with the area being expanded to about twice of its original size.
Split into simpler sentences: While Westfield Whitford City was been gradually upgraded over the years, the most extensive development occured between 2002 and 2004. With the area being expanded to about twice of its original size.