Based on your narrative, I can only guess. My guess is, that in
production, the index you mention is clustered. In that case, the data
is part of the index (IOW, the index = the table).
If you SELECT the data into a new table, and manually create an index
without the keyword CLUSTERED, then you are creating a nonclustered
index. Such an index only contains the index keys and a reference to the
clustered index or table page that stores the row, which (typically) is
just a fraction in size.