比之前变大的英文表达为 "grow bigger than before"。

"Grow" 是及物动词，意思是 "生长、发展"，例如：

The plant grew tall and strong. (这株植物长得又高又壮。)
His business has grown significantly over the past few years. (过去几年，他的生意发展得非常迅速。)

"Bigger" 是形容词，意思是 "比较大的"，例如：

She bought a bigger house because her family was growing. (因为家庭变大，她买了一栋更大的房子。)
The company is looking for a bigger office to accommodate its expanding workforce. (公司正在寻找一个更大的办公室来容纳不断扩大的员工队伍。)

"Than" 是连词，意思是 "比"，例如：

She is taller than her sister. (她比她的姐姐高。)
This car is faster than that one. (这辆车比那辆车快。)

"Before" 是副词，意思是 "以前"，例如：

He finished the project before the deadpne. (他在最后期限之前完成了项目。)
I didn't know her before we met at the party. (我在聚会时才认识她。)

因此，"grow bigger than before" 的意思是 "比以前变得更大"。例如：

The tree in my backyard has grown bigger than before. (我后院的树比以前变得更大了。)
His muscles have grown bigger than before after he started pfting weights. (他开始举重后，肌肉变得比以前更大了。)