How does Primary Index Size relate to table data size?

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How does Primary Index Size relate to table data size?

When I run this query:

select databasename, tablename, sum(currentperm) from dbc.tablesize where database name = <mydatabase> group by 1, 2 order by 2;

I receive the following size information for the table that I'm reviewing:

4,980,396,544

Then I run a query I found here to find index size for that table and the following information is returned:

IndexNumber             IndexName                 SizeinBytes

1                               tableNupi                    2,260,472,676

8                               tableNusi1                   1,184,938,920

12                             tableNusi2                   1,502,694,816

The sum of the index size is 4948106412.  That is a difference of 32,290,132

What is the relationship of the size of the Primary Index to the size of actual data stored in the table?