The majority of the memory usage is usually the data cache and the plan cache together. The memory used by individual queries is usually tiny in comparison.

Use the sys.dm_os_memory_clerks DMV, not all that understandable, but shows the various caches and their memory usage.

SELECT * FROM sys.dm_os_memory_clerks ORDER BY (single_pages_kb + multi_pages_kb + awe_allocated_kb) desc

On my test machine, that shows the largest memory consumer to be MEMORYCLERK_SQLBUFFERPOOL, the data cache, with CACHESTORE_SQLCP (ad-hoc plans) second.