I think blanket statements like sturgeons should "never" be kept in 125 gal aquaria, or "only" 10,000 l+ ponds are suitable are not helpful or accurate and are an over simplification of the housing question.
For example, i think it is more appropriate to temporarily house a newly acquired, 4"-5" fingerling in a 125 gallon tank rather than a large pond so that it can be observed more closely, make sure it's eating well, as quarantine, etc.
Placing a small fish in a large pond with other large fish would be inappropriate and arguably "inhumane" and is at the least a good way to lose a fish, or even a whole pond full of fish.
Certainly there would be a time when a sturgeon will outgrow a 125 and require larger housing. I think readers of this thread are better served by factual information so that they can make informed decisions before attempting to keep sturgeons. Emotionally charged blanket statements are not helpful, IMO.
My sterlets and diamonds have grown from approximately 5" to 8" in about 3 months, or about an inch a month.