In terms of airport size or city size?
Airports wise, MAN is 5.4 times bigger than LPL, whilst BRS is 5.7 times bigger than CWL.
In terms of city size, yes Bristol and Cardiff are similar, certainly more so than Manchester and Liverpool. But then again Bristol and Cardiff combined is 32% bigger than Liverpool, yet they have almost 250% more weekly flights to Amsterdam. Combined, the populations of Manchester and Liverpool are double that of Cardiff/Bristol, yet they only have 67% more weekly flights. Surely that demonstrates a weak link at Liverpool?
The argument of overlapping catchment I accept for long haul - it happens to pretty much every airport within 100 miles of MAN & LHR - not so sure it works so much for short haul. Sure it still comes into play, but not to such a greater extent. I'd feel OK driving over an hour to MAN/LHR for a direct long haul rather than a one stop option from my local airport. Not so OK if I was only going to Spain say.
It's more to do with city region size than actual city populations isn't it? Populations are described in so many different ways these days: municipal city; urban area; metropolitan area; city region; larger urban zone (EU).
For example, Bristol as a city, a physical city that is, is bigger than many people think. Its municipal population is currently around 450,000 but the unbroken urban area is closer to 700,000. The reason is that the city's boundaries have not been enlarged since the early 1960s but the urban sprawl has grown and grown into neighbouring areas, especially into the unitary authority of South Gloucestershire which is a 'city' with 200,000 residents physically joined - you can't see the join - to municipal Bristol. Physical (unbroken urban area) Bristol is over 50% bigger than physical Cardiff population-wise, but when it comes to the EU larger urban zone (LUZ) measurement the two are much closer because neighbouring towns and cities that are associated with the main one are taken into account.
Remaining with AMS as an example, is the difference so striking that in 2016 BRS handled 408,000 on the AMS route and CWL 135,000? BRS has 28 weekly KLM Cityhopper E190 flights as well as up to 12 weekly easyJet flights whilst CWL has 21 weekly KLM Cityhopper flights, many on the smaller E170. Actually the ratio on this route is much closer than the ratio of overall passenger numbers at these airports, but the differential still seems difficult to fathom even taking into account the size and make-up of the core and wider catchments of both.
Cardiff is also a capital city; Bristol is not.
Taking the 2011 census figures coupled with the EU's LUZ concept, Manchester/Liverpool population is over 3.5 million whilst Bristol/Cardiff is around 2 million with, it is true, a far greater bias towards Manchester whereas the Bristol/Cardiff LUZs are fairly even in population size.
In the end, I suppose it's down to airlines knowing/thinking/believing/hoping what works best for them at any given airport.