Is this the Dorset, England - CityStrides you’re referring to?
There’s A Whole Thing™ about many places in England - Replace England's admin level 6 (and 8 in some cases) with admin level 10
The community is (as far as I remember) quite divided on the issue. A handful of people contact me about an area too large existing in CityStrides … I replace it with the smaller areas … A handful of other people contact me about an area too small existing in CityStrides 
If you’re a Supporter, you can zoom out a bit to see all fo England & enable the city border view (globe icon button on the map) … you’ll see areas with more space e.g. Dorset … and you’ll see areas with smaller borders e.g. Wells just a bit northwest from Dorset.
If you click the top left “Run” button here - overpass turbo - you can zoom in and click on the displayed cities to get a sense of what would replace Dorset (this includes your example Weymouth).
I’d urge you (and anyone else reading along who’s local) to click around on a few places and check to see if they all seem more representative of “city” (as loaded of a term as it is) … maybe Dorset feels more like a county (from my own US perspective) and places like Winterborne Stickland feel more relevant
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I think I usually consider this from the perspective of “What do you say when people ask you where you’re from?” … if you say “I’m from Dorset” then, we’re done here - apologies for the number of streets ahead of you 
And I realize this isn’t a perfect approach, either … from the more pedantic side of there being likely 0 people saying they’re from “Dorchester Town Council” as opposed to just Dorchester … and 
maybe people from Arne say they’re from Wareham … since this situation is definitely not a one-size-fits all, I’m just looking for the least worst option 