The reason why we are seeing Inter as favorites is because of last year, and because of historical reasons. Historically speaking, Napoli won recently, but before that it has been years since they won last time, Inter has always been a big team, and even when Juventus was dominating the league, we have seen Inter do fine, they were either second or third, and always been good, whereas Napoli was volatile and did badly at times.
This is why, many people believe that Napoli do not have what it takes to win it at the end, we should be seeing some changes there without a doubt if the gap is even bigger. If Napoli wins their game, the gap would be 4 points, and that would be a lot better for them without a doubt.
You are right, Inter Milan always has a big name and with a good history too, if compared to Napoli then we will find a lot of differences. Maybe some people will say that it is just history and what happens now could be different. That is not wrong, but one thing we can see is their journey too. When a team with a long journey they are always in tight competition and also win several titles, then mentally they will also be more trusted. This is a habit and in fact it happens like that. Not to underestimate Napoli, they are also a good team, but here we are just having an opinion.