how to explain this result

My developers turned me a query and claimed this query takes 6 minutes to run under production instance and almost 30 minutes to run under development instance. I then check the execution plan in both instacnes, superisely they are identical. By using sql trace and tkprof, I saw number of "disk" read is 22478 in production instance and 191580 in development. The only difference I could
think of is SGA sizes are not same (160 M in production and 32 M in development). To prove that number of "disk" read is caused by SGA size, I increased SGA (db_block_buffer) to 160 M in my development instance and run the same query again, the number of "disk" read dropped significantly to 22478.

The question I have here is why db_block_buffer size has so much impact on number of disk read?

Any helps would be highly appreciated.

• explain plan ********* Rows Execution Plan ------- --------------------------------------------------- 0 SELECT STATEMENT GOAL: CHOOSE 372 FILTER 372 NESTED LOOPS 324818 TABLE ACCESS GOAL: ANALYZED (FULL) OF 'PS_JOB' 286339 TABLE ACCESS GOAL: ANALYZED (BY ROWID) OF 'PS_POSITION_DATA' 784772 INDEX GOAL: ANALYZED (RANGE SCAN) OF 'PS_POSITION_DATA' (UNIQUE) 497139 SORT (AGGREGATE) 782814 INDEX GOAL: ANALYZED (RANGE SCAN) OF 'PS_POSITION_DATA' (UNIQUE) 481 SORT (AGGREGATE) 853 INDEX GOAL: ANALYZED (RANGE SCAN) OF 'PSAJOB' (NON-UNIQUE)