Disk busy after gather statistic big table

 Hi,

Could you please help explain and suggest me why disk busy after gather statistics on Big Table? I try to collect statistics on my database using dbms_stats.gather_table_stats for small table it seem ok, after collect statistic on big table(10+ gb), most of statement hit that table waiting for I/O.

information:

-My database version is 9.2.0.8. 

-small table size < 1 gb

-big table >10 gb

-gather option 5%, casdcade=> true

Thank you.

Nim.