I have been playing with various guest memory sizes and came to a conclusion that the description of WEI for memory performace is a bit misleading. I thought that the only criteria for the memory performance would be a number of operations per second, but it looks like the WEI scoring algorithm takes absolute memory size into account as well. Here are the WEI values for different guest memory sizes (no other changes have been done to the guest VM):
Guest VM RAM Size (WEI Score)
1.5 GB (4.5)
2 GB (5.5)
4 GB (7.5)
So, I guess the above answers my question.