How to Find Actual Data Size from a Query in SQL Server

1. Check that if you have indexes (clustered/nonclustered, full text). Use sp_spaceused 'your_table_name' to check that you can actually discard indexes.
2. Check what type of table is used. In your example of 0.2KB=205 Bytes you will have 38 rows per data page if your table is heap and 39 rows per data page if it is clustered table.

Please see below the example:

IF EXISTS (SELECT * FROM sys.tables
            WHERE name = 'sparse_pages')
    DROP TABLE sparse_pages;
GO
CREATE TABLE sparse_pages 
(
KeyField SMALLINT --IDENTITY (1,1) PRIMARY KEY
, Filler VARCHAR(8000) null 
)
GO

SET NOCOUNT ON
INSERT INTO sparse_pages( Filler) values ( REPLICATE('a', 192))
GO 39

-- Average row size now 205 Bytes
WAITFOR DELAY '00:00:03';
GO
SELECT 'This is Heap. Note Data Space'
GO
-- Check table size
sp_spaceused 'sparse_pages'
GO


IF EXISTS (SELECT * FROM sys.tables
            WHERE name = 'sparse_pages')
    DROP TABLE sparse_pages;
GO
CREATE TABLE sparse_pages 
(
KeyField SMALLINT IDENTITY (1,1) PRIMARY KEY
, Filler VARCHAR(8000) null 
)
GO

SET NOCOUNT ON
INSERT INTO sparse_pages( Filler) values ( REPLICATE('a', 192))
GO 39

-- Average row size now 205 Bytes
WAITFOR DELAY '00:00:03';
GO
SELECT 'This is Clustered Index. Note Data Space'
GO
-- Check table size
sp_spaceused 'sparse_pages'
GO

3. Check for random inserts/updates/deletes in your table. This may be an issue as free space is not reclaimed back. The free space may be wasted during page splits too.

Please see below the example with average row size 205 Bytes and 200 rows (just like in your case). Table data size is 1.57 MB:

IF EXISTS (SELECT * FROM sys.tables
            WHERE name = 'sparse_pages')
    DROP TABLE sparse_pages;
GO
CREATE TABLE sparse_pages 
(
KeyField SMALLINT IDENTITY (1,1) PRIMARY KEY
, Filler VARCHAR(8000) null 
)
GO

Enter the data

SET NOCOUNT ON
INSERT INTO sparse_pages( Filler) values ( REPLICATE('a', 8700))
INSERT INTO sparse_pages( Filler) values ( REPLICATE('a', 192))
GO 200

DELETE FROM sparse_pages 
WHERE LEN(Filler)>300
GO
-- Average row size now 205 Bytes
WAITFOR DELAY '00:00:03';
GO
-- Check table size
sp_spaceused 'sparse_pages'
GO

You don't mention what engine this is going against so I'm assuming it's Microsoft SQL Server. One word of advice is that you put the join predicate and specify the type of join in the FROM clause, it will make errors like this much more visible as well as making reading the query easier. You forgot to specify the join type for the db1.dbo.Products table, which I think makes it a cross join, so vastly multiplying the number of rows you would get back. See below.

merge into factOrders fo
using (
    select CustomerKey
        ,ProductKey
        ,CategoryKey
        ,dt1.TimeKey as [OrderDateKey]
        ,dt2.TimeKey as [RequiredDateKey]
        ,dt3.TimeKey as [ShippedDateKey]
        ,o.OrderID
        ,Freight
        ,od.UnitPrice as [UnitPrice]
        ,od.Quantity as [Quantity]
        ,od.Discount as [Discount]
        ,od.UnitPrice * od.Quantity * (1 - Discount) as [TotalPrice]
        ,sh.CompanyName as [ShipperName]
        ,sh.Phone as [ShipperPhone]
    from db1.dbo.Orders o
        INNER JOIN db1.dbo.[Order Details] od ON o.OrderID = od.OrderID
        INNER JOIN db1.dbo.Shippers sh ON sh.ShipperID = o.ShipVia
        INNER JOIN dimCustomers dc ON dc.CustomerID = o.CustomerID
        INNER JOIN dimProducts dp ON dp.ProductID = od.ProductID
        INNER JOIN dimCategories dct ON dct.CategoryID = p.CategoryID
        INNER JOIN db1.dbo.Products p ON 
        INNER JOIN dimTime dt1 ON dt1.date = o.OrderDate
        INNER JOIN dimTime dt2 ON dt2.date = o.RequiredDate
        INNER JOIN dimTime dt3 ON dt3.date = o.ShippedDate
    ) o
    on (
            fo.ProductKey = o.ProductKey
            and fo.CustomerKey = o.CustomerKey
            and fo.CategoryKey = o.CategoryKey
            and fo.OrderDateKey = o.OrderDateKey
            )