It's just a feeling IMHO, based on the car, equipment level, size, car width, and car it is based on.

i.e. while the City may be pretty big for a B segment car, it is based on the Jazz. But with a boot. Car width is a bit of a giveaway, as that hardly changes.

For example the Golf 7 (excluding mirrors) is 1,80 wide. The City is 1,69 wide.

Also it's IMHO based on history... the name plays a role too. i.e. City was always a B segment sedan or hatchback. The Golf has always been a C segment car, even though the first generation would, in terms of size, be more like A or B segment.

Then there is the price, expected quality levels (well, how it feels inside)... the higher the segment, the posher the car.

There are no hard rules which car belongs to which segment, for example you could consider the Skoda Octavia as a big C segment car, or a small D segment car. IMHO I'd go with C segment, as it was always based on the Golf (in times of the MQB platform and similar ones from other brands this got muddled up too...), and by association thus it also is C segment. Plus the price is closer to C segment.

Proton poses in interesting problem. IMHO Preve and Suprima are C segment, and so were Persona and Gen-2 IMHO. Others may say they are B segment, but for their time they were IMHO too big for that. In terms of price they'd all be B segment though. But the new Persona, being based on the Iriz, may fall into the B segment.

Oh, and... who says the Subaru XV isn't a crossover? IMHO it absolutely is. SUVs are a cross between proper offroaders and normal road cars, but heavily leaning on the offroader side. Crossover are the same, but leaning towards road cars. As such it absolutely should be in that category.