Guy Glantser has a script to track buffer pool size by table:
Sometimes we need to troubleshoot memory pressure issues in SQL Server or in Azure SQL. One of the things that can help in these cases is to view the contents of the buffer pool.
I wrote a simple script that displays the contents of the buffer pool in terms of tables in the current database. For each table, it presents the total table space and the space consumed by the table in the buffer pool. The script is based mainly on the sys.dm_os_buffer_descriptors dynamic management view.
Click through for the script.