The balloon became fully distended after being inflated.

distended 是动词 distend 的过去分词形式。distend 由前缀 dis-（表示"向外"）和拉丁词根 tendere（"伸展"）组成，意思是"向外伸展、膨胀"。distended 在这里描述气球被充气后完全膨胀开来的状态。