Salaamun Alaykum

Watching pornography would go against the clear command of Qur'an to "lower the gaze", Suratul Noor 24:30-31.

What actually entails a major sin, and what the punishments for those are, is a much discussed and debated question.

I understand that it could be interesting to know whether or not watching pornography would fall into such a category.

However, I would like to highlight the issue from another perspective. Generally, when something is haraam in sharia, it is so for good reasons. There is an abundant amount of evidence showing that pornography is harmful for the persons involved in the filming, for the individual watching, and for the society at large. 

Many books have been written on the issue, e.g. Understanding and treating Sex and Pornography Addication by Paula Hall, and Treating Pornography Addiction by Kevin B Skinner.

I would also recommend the following TEDx talk: The Great Porn Experiment 

To cut it short, pornography is highly addictive. Furthermore, it can cause serious problems in life in different domains, such as marital intimacy, concentration, carrier, and other areas. 

Therefore, I would suggest that one looks beyond the question of major sin or not. The fact is that it is potentially very harmful. These are the dire consequences in this life. I dread to think about the consequences on the soul and the life to come.

May God bless you