Memory issue for inheritance tables.

One parent table, having 100 child tables.
In this scenario we observed that delete and update are taking more memory for preparing a plan

If the system under peak i am getting out of memory issue.

i.e
Select on parent table is using memory -  331456 in message context
Delete on parent table is using memory - 3746432 in message context

Delete on single child table is using memory - 8800 in message context
Select on single child table is using memory - 9328 in message context

For 250 child tables
        Select on parent table is using memory -   810864 in message context
        Delete on parent table is using memory - 21273088 in message context

I had seen the plans for both delete & select on parent table almost same.
why this much of memory increase in message context for delete and update operation.

Regards,
Hari babu.