How common is it for a dentist to put an unnecessarily large filling inside a tooth - thus creating a risk that the tooth may end up needing a root canal somewhere down the line?

It is not common, that is not what dentists do. When a tooth needs a filling, or a filling replaced we remove undermined and damaged tooth stucture so that the restoration we place can seal off the damaged area and save the tooth. Most often we prevent the tooth form needing root treatment. If because of neglect you need root treatment be grateful it can be done for you.

Why would your dentist place an unnecessarily large restoration in your tooth? All decay needs to be removed, and if the lesion went undiagnosed for a long time then there is a chance the tooth will need further treatment. That's why it is important to have exams and cleanings every six months in order to help prevent these issues.

As a dentist I inform my patients that they have a large and deep cavity near the nerve, and I will try to save the tooth with a big filling and a protective lining. I also inform them that a possible root canal and or a crown may be needed in future if pain and or infection arises. The alternative is an extraction and a more expensive treatment.