Why are tiny Innodb row updates causing so much disk IO?
Any UPDATE to a row requires the following (sooner or later):


Read the 16KB block containing the row (unless cached in the buffer pool)
Write the block back to disk (or cache it a long time)
The "doublebuffer" is written once per transaction
Do one write to the InnoDB log for transactional integrity.  (The
details here depend on various settings and whether you wrap things
in BEGIN..COMMIT.)
Since you say the buffer_pool is big enough (apparently for the entire table and any secondary indexes?), the first two items will not tally many IOPs.


But that was not your question.

The actual I/O (pages) related to the buffer_pool is (Innodb_buffer_pool_reads + Innodb_buffer_pool_pages_flushed) 

You will probably find this very low because of good caching:
Innodb_buffer_pool_reads / Innodb_buffer_pool_read_requests