You make some good points, and to be clear, I am in no way blaming the market size on the Reds failures at any time. I've made the claim numerous times that they can and need to spend over double what they are currently spending on payroll.
But two key facts are clear. 1. The Padres are a bigger market than the Reds. 2. The Padres have not been more competitive than the Reds historically.
Here are the facts, the history of both teams, and a quick glance reveals that the Padres have an advantage over the Reds attendance wise, no matter the situation, and their records have not been better than the Reds, since the Padres have existed. It's not even true if you go back over just the past 25 years.
https://www.baseball-reference.com/teams/CIN/
https://www.baseball-reference.com/teams/SDP/
Let me point out a few key facts that are in these links.
First, record wise, since the 1994 season cancellation (thanks to WOY for this):
Reds - 2117-2300
Padres - 2122-2295
Each have had 5 playoff seasons.
Second, The best the Reds have ever done attendance wise, since the Padres have existed is 1976 with 2.6M. The best the Padres have ever done attendance wise is 3M.
Since 1994, the Reds have had 13 seasons with over 2M attendance. Since 1994, the Padres have had 27 seasons with over 2M attendance.
Similar records. Very dissimilar attendance numbers.