扩大 is the general-purpose verb for making something larger in size, scope, or scale — e.g., 扩大业务 (expand business) or 扩大面积 (expand area). 扩展 and 拓展 focus on extending reach or scope (like 扩展知识面 broaden knowledge); 扩张 is more territorial or physiological (扩张领土 expand territory, 扩张血管 dilate blood vessels); 扩建 is specifically for buildings and facilities.