Judging everything by the standards of Dark/Middle Age Europe is a very narrow point of view to take in a game that includes elves, dwarves, tieflings, "wombfreaks," half-orcs, etc.
The reason ten or more of these "exceptional individuals," i.e. abnormally-sized humans, can be found gathered in the outskirts on a daily basis is simply that the Mists chose those "more than a hundred" people based on the fact that they WERE exceptional, and they now live and congregate amongst the tens of thousands of Core residents, who ALSO have their own exceptional individuals to add to the mix. The fact that they only have a few "safe" places to gather means that they will gravitate to those areas and be found there en masse.
In fact the reasoning does NOT fall apart on a NWN server...it becomes more valid.
Besides....you are all overstating the problem. I look at individuals' descriptions on a near-constant basis (usually while waiting for people to get done typing in RP), and I would say 50% of them say nothing except "Uninjured/Neutral," and of the remaining 50% that I see less than 20% of those actually list a height. Most are just simple physical descriptions, some are page-long narrative histories. Yes, a lot of the people who put heights in their descriptions would be considered "abnormal," but the rest of the people WITHOUT a description or a height could easily be considered to fall within the average, yes?