As Wellsy says, the 'town' proper does only have around 90k,
in the same way the 'City of Sydney' only has around 200k. The borough of Wigan, which is basically the surrounding metropolitan area, has around 300k, and people from there would identify to outsiders as being from Wigan. Somewhere like Orrell is only 3 miles from Wigan town centre, the people from there are definitely from Wigan, but they wouldn't have been counted on population figures for Wigan in the past because traditionally it's not part of the 'town'. Because England is so old, places weren't ever properly incorporated into cities, so the whole thing is a bit messed up and it's all a bit unclear where towns begin and end. The place that I would describe as Wigan is the borough, trying to narrow things down to the traditional town boundaries seems pedantic and pointless to me, and also doesn't give an accurate picture. It's just tradition I guess. That's why you can't find a decent list of UK cities by population on Wikipedia. If you came to Wigan, you would find it's a pretty big place with quite a large population, rather than just an isolated town of 90k like you would find in rural Queensland or somewhere.
As you can see from
this map, Salford is pretty much part of Manchester. Except it's not, it's technically a city in its own right.