How much memory is a table taking up?
with bd as (
    select count(*) as pages_in_memory, bd.allocation_unit_id
    from sys.dm_os_buffer_descriptors bd
    where bd.database_id = db_id()
    group by bd.allocation_unit_id)
select p.object_id,
    p.index_id,
    p.partition_number,
    bd.pages_in_memory,
    au.total_pages as pages_on_disk,
    au.type_desc
from bd 
join sys.allocation_units au 
    on au.allocation_unit_id = bd.allocation_unit_id
join sys.partitions p
    on p.partition_id = au.container_id