The total market for AAA games right now is probably in the region of 250 to 300 million people if you look at the install base of xbox, playstation and pc.
With gamepass being on pc, xbox and cloud they could potentially capture a substantial part of that market and even expand it due to the low barrier of entry of xcloud or their rent to own scheme. They have 15 million subscribers now which is about 2 billion in revenue depending on the composition of their subscribers. They grew 50% in just 6 months. The potential for gamepass is therefore vast compared to traditional boxed products. If they capture 50% of the current market, thats 20 billion. That would be similar to netflix in size.