The increase in memory usage could be due to the 'auto-tuning'
functionality that was introduced in Web Server 7.0U2. (But not yet documented)
This feature will set some of the servers parameters based on number of CPU's,
file descriptors, etc of the actual machines.
It is possible to see what these are being auto-tuned to by setting the
log level to finest, and restarting each server.
They should see differences between the two boxes.
The one best thing that can be done is run perfdump to see how much of
the set resources are actually being used. Once you have those figures, you
can then manually set these values in the server.xml file, which will
override any auto-tuning values.
Web Server 7.0 will definitely require more memory than 6.0,
basically because it's architecture is very different.
So in recap you would want to start by setting the log level to finest
on both servers, enabling perfump and restarting them. Then gathering
some perfdump data as the memory usage reaches its high levels. This should
allow some thoughts for settingl initial values you might want to set
these parameters to.