Shamelessly lifted from http://www.sqlteam.com/article/finding-the-biggest-tables-in-a-database, with thanks.  😀

/**************************************************************************************
*
*  BigTables.sql
*  Bill Graziano (SQLTeam.com)
*  graz@sqlteam.com
*  v1.1
*
**************************************************************************************/

declare @id int  
declare @type character(2)  
declare @pages int  
declare @dbname sysname
declare @dbsize dec(15,0)
declare @bytesperpage dec(15,0)
declare @pagesperMB  dec(15,0)

create table #spt_space
(
objid  int null,
rows  int null,
reserved dec(15) null,
data  dec(15) null,
indexp  dec(15) null,
unused  dec(15) null
)

set nocount on

-- Create a cursor to loop through the user tables
declare c_tables cursor for
select id
from sysobjects
where xtype = 'U'

open c_tables

fetch next from c_tables
into @id

while @@fetch_status = 0
begin

 /* Code from sp_spaceused */
insert into #spt_space (objid, reserved)
  select objid = @id, sum(reserved)
   from sysindexes
    where indid in (0, 1, 255)
     and id = @id

 select @pages = sum(dpages)
   from sysindexes
    where indid < 2
     and id = @id
select @pages = @pages + isnull(sum(used), 0)
  from sysindexes
   where indid = 255
    and id = @id
update #spt_space
  set data = @pages
where objid = @id

 /* index: sum(used) where indid in (0, 1, 255) - data */
update #spt_space
  set indexp = (select sum(used)
    from sysindexes
    where indid in (0, 1, 255)
    and id = @id)
       - data
  where objid = @id

 /* unused: sum(reserved) - sum(used) where indid in (0, 1, 255) */
update #spt_space
  set unused = reserved
    - (select sum(used)
     from sysindexes
      where indid in (0, 1, 255)
      and id = @id)
  where objid = @id

 update #spt_space
  set rows = i.rows
   from sysindexes i
    where i.indid < 2
    and i.id = @id
    and objid = @id

 fetch next from c_tables
into @id
end

select top 25
Table_Name = (select left(name,25) from sysobjects where id = objid),
rows = convert(char(11), rows),
reserved_KB = ltrim(str(reserved * d.low / 1024.,15,0) + ' ' + 'KB'),
data_KB = ltrim(str(data * d.low / 1024.,15,0) + ' ' + 'KB'),
index_size_KB = ltrim(str(indexp * d.low / 1024.,15,0) + ' ' + 'KB'),
unused_KB = ltrim(str(unused * d.low / 1024.,15,0) + ' ' + 'KB')

from  #spt_space, master.dbo.spt_values d
where  d.number = 1
and  d.type = 'E'
order by reserved desc

drop table #spt_space
close c_tables
deallocate c_tables