How about increasing default value to 32KiB or 64KiB too?

binutils's cp works well for most cases, while it doesn't have option to specify blocksize.
I want Python's copy functions works nice enough for common cases too.

binutils cp uses 128KiB block for normal file. see more information in https://github.com/coreutils/coreutils/blob/master/src/ioblksize.h

But copyfileobj can be used other file-likes.
On Linux, typical socket buffer size is 128KiB, and typical pipe buffer size is 64KiB.  So block size larger than 64KiB may cause unneeded blocking.
That's why I suggested 32KiB or 64KiB.