This is good stuff guys, thanks for weighing in. What I think Im learning is there isn't a real theory on this stuff. Its probably more about real estate and percieved marketability than anything.
As far as what I question, it's more when a manufacturer could but they don't, like the 10mm example being smaller than .45 and likely there being enough room in the handle for an extra round but they pass on it. I figured there must be a good reason, like a military or law enforcement specification even maybe.
On most guns though I'd say within reason there is a market for extended mags, seems like most immediately go hunting around for a little extra capacity after any purchase. Take an SR22 for example, lots of us go and spend the $$ and effort to add 2 kits to our mags to bump them up to 17 rounds from 10, yet Ruger seems to take no notice and instead invests in color schemes and threaded barrel options
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