SQL Server clustered index bigger than actual table. Why and how do I fix it?

I have a few tables in a SQL Server database where the clustered index is really big, up to 50% of the table size, just for the ID's?

For example :

• Table 1 30 million rows, 10GB of data, clustered index = 10GB
• Table 2 40 million rows, 2.4GB of data, clustered index = 18GB

The script to get the size of the indexes I am using is : https://blog.sqlauthority.com/2016/11/13/find-size-indexes-database-interview-question-week-097/

SELECT
    OBJECT_SCHEMA_NAME(i.OBJECT_ID) AS SchemaName,
    OBJECT_NAME(i.OBJECT_ID) AS TableName,
    i.name AS IndexName,
    i.index_id AS IndexID,
    8 * SUM(a.used_pages) AS 'Indexsize(KB)'
FROM 
    sys.indexes AS i
JOIN 
    sys.partitions AS p ON p.OBJECT_ID = i.OBJECT_ID AND p.index_i = i.index_id
JOIN 
    sys.allocation_units AS a ON a.container_id = p.partition_id
GROUP BY 
    i.OBJECT_ID, i.index_id, i.name
ORDER BY 
    8 * SUM(a.used_pages)

The index code is as follows (the same for both tables) :

ALTER TABLE [dbo].[my_Table] 
     ADD PRIMARY KEY CLUSTERED ([pkID] ASC)
         WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, 
               SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, ONLINE = OFF, 
               ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO

Please can someone assist me in fixing this or help explain why this has happened?

The indexes are not fragmented at all (<5%), surely as the clustered index, it just has the PK in it? Each table has a nvarchar(max) in it, would that be a "hidden" include in the index? Still does not explain as to why it is larger than the actual table though?

I am using SQL Server 2016 Standard.

Thank you have and have a great day. Wade