Highly Memory Usage

[Zi55]

Hello Everyone ,
I have a forum with about 900K members ,177K Threads and 18M posts .
My problem , I have about 8-15 Query on every page , but the problem is every page usage a high memory between 12M-15M without any reason , even if i switch the all plugins off , the problem is still in my forum , the problem from the data store i think , because in every page it have the largest number of memory use ( as i see in query explain page)
Time Before: 0.00540 seconds
Time After: 0.01650 seconds
Time Taken: 0.01110 seconds

Memory Before: 3,951.320 KB
Memory After: 8,155.016 KB
Memory Used: 4,203.695 KB
What i really hope is that you help me to fix my problem , because i try to compare my forums with smilier forums , they didn't use more then 3MB of ram , but my forum always use a high number of memory

Thanks ,
Zi55