Is it normal for the buffer cache size to vary throughout the day?
When I monitor my SQL Server, I can see that the buffer cache size (number of pages) grows through out the day as the number of user connections increase. This cache size varies between 2-3MB to 30-35MB. The server has 256MB and is dedicated to running SQL Server. The buffer cache hit ratio is 99.5%, so I am assuming everything is running OK.
The behavior you are seeing is normal. As more users access SQL Server, SQL Server will increase the buffer cache as necessary to ensure a high hit ratio (assuming there is RAM to do so).
If your buffer cache ratio is over 90%–as it is in your case–can you can be sure that SQL Server has all the RAM it needs to run as efficiently as it can.