what you can do is

1. Find out the database that consumes highest memory in buffer pool using below :

   SELECT COUNT(*) AS cached_pages_count , 
       ( COUNT(*) * 8.0 ) / 1024 AS MB , 
       CASE database_id 
         WHEN 32767 THEN 'ResourceDb' 
         ELSE DB_NAME(database_id) 
       END AS Database_name 
   FROM sys.dm_os_buffer_descriptors 
   GROUP BY database_id
   

2. Drill down the database using below query :

   set nocount on;
   set transaction isolation level read uncommitted;
   select
      count(*)as cached_pages_count,
      (COUNT(*) * 8.0) / 1024 AS Total_MB_Occupied, -- convert pages into MB -  the page size is 8 KB for sql server
      obj.name as objectname,
      ind.name as indexname,
      obj.index_id as indexid
   from sys.dm_os_buffer_descriptors as bd
   inner join
   (
       select       object_id as objectid,
                          object_name(object_id) as name,
                          index_id,allocation_unit_id
       from sys.allocation_units as au
           inner join sys.partitions as p
               on au.container_id = p.hobt_id
                   and (au.type = 1 or au.type = 3)
       union all
       select       object_id as objectid,
                          object_name(object_id) as name,
                          index_id,allocation_unit_id
       from sys.allocation_units as au
           inner join sys.partitions as p
               on au.container_id = p.partition_id
                   and au.type = 2
   ) as obj
       on bd.allocation_unit_id = obj.allocation_unit_id
   left outer join sys.indexes ind 
   on  obj.objectid = ind.object_id
   and  obj.index_id = ind.index_id
   where bd.database_id = db_id()
   and bd.page_type in ('data_page', 'index_page')
   group by obj.name, ind.name, obj.index_id
   order by cached_pages_count desc
   

Reference :

• A quick look at: sys.dm_os_buffer_descriptors
• What's swimming in your bufferpool?
• sys.dm_os_buffer_descriptors aggregations -- more detailed info - what objects/structures/files/etc. are consuming the largest amount of space at a given time (or over time) within the Sql Server buffer pool.