Query that consumes a lot of memory

Hello, I am new in this forum.  I have a problem and I hope you can help me.

I have a query, that reserves more than 20 Gb of memory, and uses only a few Mb.  I don't understand why the optimizer choose to reserve this amount of memory, the tables involved has their statistics up to date. 

I am watching the values in view sys.dm_exec_query_memory_grants.  I have for example in one execution this values:

requested_memory_kb: 20241384

used_memory_kb: 3072

ideal_memory_kb: 32648960

Is any wrong in my analyze?  Can you help me to understand this situation please?.

Thank you so much.

Regards!!