Van Heghe Eddy:

Keep in mind that when SQL creates an index on a table, there are basically two types of pages in that index: upper-level (non-leaf) index pages and lowest-level (leaf) index pages. The non-leaf level(s) store only key values, not raw data from the table. The leaf pages store all data from the index, including any included columns in a nonclustered index.

When SQL shows you "index" size for a clustered index, it's the size of the non-leaf pages only. But when SQL rebuilds a clustered index, it has to use the entire row, including leaf pages.