Your Question 1):
If a Clustered index is not specified, doesn't SQL Server create its own clustered index behind the scenes?
answer:
if you miss to specify the Index Type, there is no implicit speicifation from Mircosoft to create a Clustered/NonClustered Index behind the screen as it is purely depends on the user and requirement of the table creation, so simply it will be a "Heap" as you told.
2) Why is the NonClusterered table (with a primary key) versus the Clustered table (with a primary key) about 2x the size?
answer:
as like "Gail" & "Steve Jones" told you, Non Clustered Index would occupy more space as it holds one Index and one Row Identifier
whereas the clustered index dont have that structure so it will occupy less space and faster too..
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I believe I have just given my opinion and not more anything better than the three experts above (Steve,Grant,Gail)
Regards,
Prabhu