大型二维静态数组和向量向量的内存使用

I need to use large matrix 20000 * 20000 for a machine learning project. When it is implemented as static array, it uses approximately 1.57 GB of memory. If it is implemented with vector of vectors it uses much more memory then the static array (approximately 3.06 GB). I could not figure out the reason behind it.I need to use large matrix 20000 * 20000 for a