Sure, but the point is you will get different results depending on which metric you pick. It's not that one is inherently more correct than the other.

Often there are cultural aspects at play too. I live in London. I also live in Croydon - it'd be one of the largest cities in the UK in its own right if it was separate to London (and on more than one occasion the council has tried to make that happen). Most of the borough is part of the same urban area, but some are fairly well separated. Several parts of the borough are really separate towns, separated not just from London but from Croydon by "relatively rural" land (by our standards), with their own town centers, train stations, and culturally distinct.

All of this is within the administrative city limits of Greater London ("London" doesn't really exist as an entity of its own - and before anyone else says City of London, that's one tiny constituent part of Greater London - nobody means City when they say London)

So when you say Berlin, everything might be within city limits, when you say London, odds are you wouldn't think of every part within the actual city limits as part of it, nor even a city, but the Greater London urban area includes tendrils extending beyond Greater London too that nobody other than perhaps extra audacious real estate agents would call London. When you say Paris, odds are you might include some parts beyond the formal city limits but very unlikely the entire urban area.

Even then, you won't even get people living each place to agree where the line goes.