Clustered Index / Index Oriented Table performance in join, worse than nonclustered?
You are correct Erik.
In case a large portion of the leaf level of the clustered index needs to be read, the size of the other columns affects the amount of data that needs to be read, since the leaf level of a clustered index contains the table pages.

Nonclustered indexes contain the clustered index values for the ability to perform a lookup when a column that is not in the nonclustered index needs to be fetched. The optimizer can leverage this in order to fetch the clustered index values from there when it decided it's cheaper to do that.