How do Kernel use pagecache?

I have a problem with pagecache that I don't understand. As I understand, the pagecache will serve as disk cache for reading from and writing to disk. But I don't know how the kernel can map 10G memory pagecache to 200GB disk. When reading, the kernel can only cache data that is regularly read from disk, but when writing to disk, data will be written to memory buffer and then the kernel will write to disk. But how can memory have enough buffer when the data written to disk is very large?