A typical buffer size for DOS was 512 bytes, the size of a floppy disk sector. I used 20K. Writing bigger chunks at a time enabled the operating system to optimize it for better throughput to the disk driver. (Fewer seeks, for example.)

Fewer round trips to the operating system almost always speeds things up, too, especially if it involves a context switch.

Things got even better when the switch to hard disks was underway. Hard disks had bigger sector sizes, but the buffer size sometimes remained at 512. This meant the driver had to read the sector, fold in the 512, then write it back out. Oops!

Later on, demand paged virtual memory and memory caching also favored big buffers.