Large amount of projected I/O with Oracle, even if only a single record is fetched

I often encounter the following situation in my Oracle execution plans:

Operation                   | Object  | Order | Rows | Bytes | Projection
----------------------------+---------+-------+------+-------+-------------
TABLE ACCESS BY INDEX ROWID | PROD    |     7 |   2M |   28M | PROD.VALUE
  INDEX UNIQUE SCAN         | PROD_PK |     6 |   1  |       | PROD.ROWID

This is an extract from a larger execution plan. Essentially, I'm accessing (joining) a table using the table's primary key. Typically, there is another table ACCO with ACCO.PROD_ID = PROD.ID, where PROD_PK is the primary key on PROD.ID. Obviously, the table can be accessed using a UNIQUE SCAN, but as soon as I have some silly projection on that table, it seems as though the whole table (around 2 million rows) is planned to be read in memory. I get a lot of I/O and buffer gets. When I remove the projection from the greater query, the problem disappears:

Operation                   | Object  | Order | Rows | Bytes | Projection
----------------------------+---------+-------+------+-------+-------------
TABLE ACCESS BY INDEX ROWID | PROD    |     7 |   1  |     8 | PROD.ID
  INDEX UNIQUE SCAN         | PROD_PK |     6 |   1  |       | PROD.ROWID

I don't understand this behaviour. What could be the reasons for this? Note, I cannot post the complete query. It is rather complex and involves a lot of calculations. The pattern, however, is often the same.

UPDATE: I maganged to bring down my rather complex setup to a simple simulation that produces a similar execution plan in both cases (when projecting PROD.VALUE or when leaving it away):

Create the following database:

-- products have a value
create table prod as
select level as id, 10 as value from dual 
connect by level < 100000;
alter table prod add constrai