Co-owner David Sullivan has blasted Birmingham's "armchair fans" who only turn out for high profile matches.
The Blues only managed to sell 13,700 season tickets after securing their return to the Premier League.
And Sullivan said: "We are competing with many clubs who are bigger in terms of capacity and season ticket holders.
"The reality is that most of our supporters are armchair fans unless it is a big game and what we have sold is simply not enough."
City attracted a crowd of 24,898 for their opening match against Sunderland which went up to 24,981 for the 1-0 home defeat to West Ham on Saturday.
And Sullivan admitted: "Many Championship clubs get bigger gates than us.
"The other promoted clubs, Sunderland and Derby, have 32,000 and 28,000 season ticket holders respectively.
"We all know that we have one of the biggest fan bases in the country and, if they turned out and supported us every week, we would need a 60,000 capacity stadium.
"Evidence of this was the Auto Windscreen Shield final a few years back when we took the largest following any club had taken to the old Wembley since the War (around 48,000).
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"But the reality is we have not sold enough even though our season ticket prices and match day admission prices are the second lowest in the division."
Meanwhile, Sullivan insists he and chairman David Gold would only relinquish power at St Andrew's when the time is right.
Hong Kong businessman Carson Yeung has bought a 29.9% stake and is understood to want to take full control within the next couple of months.
Sullivan said: "I know some people are concerned about what is going on with the club's share-holding but I want to assure them there have been no changes.
"The club is being run by the people who have done a great job for the past 14-15 years and therefore the club is in good hands.
"David and I will only cease to be majority shareholders if we think someone can do a better job of running Birmingham City than we have."
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