How to find memory usage by table? I want to discover how much memory is being used by each table. Is this information available in DMVs or elsewhere?
what you can do is 


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

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.