Websphere Performance tuning

Helo Experts,

We have recently mugrated our application from Weblogic 7.0 to Websphere 6.1.

We started the websphere server with teh same setting as weblogic but the server started throwing Out Of Memory error continuously.
The error was resolved only after increasing the Java heap size to 4GB.

Same application/user base was running in weblogic with 2GB (heap size) which never threw this memory error.

Really not sure why websphere neeeds more memory.
The application behavior is slower in WAS compared to Weblogic,.The concern is if there is any other attribute4 that needs to be altered in WAS to handle performance related issues?

Please share your experience with performance enhancement in WAS server.

Thanks
Viswa