Why Table buffer size is larger than its physical size?

Hi,

 I'm trying to troubleshoot database peformance and when I'm trying to look at table buffer sizes then I found that a table in the buffer consuming more than 5 GB of the memory. But the fact is, physical size of the table is actually only 2 GB.

 I didn't understand why buffer size of a table will be larger than the actual physical size of the table.

 Any great ideas?

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