Is it possible to have been born with a larger than normal bladder and then be mis-diagnosed with bladder retention ?

No,: Although functional bladder capacity varies among people, they should be able to empty their bladder. Of no doubt, you have noticed progressive slowing urine flow resulting in having to stand closer ; closer to urinal and taking longer time to finish a peeing. Fact: people accept slowing flow comes with age and wouldn't seek attention until having enough personal inconvenience...So, listen to doc.