The size scale of ships isn't utterly arbitrary, as I pointed out in an earlier post. While you can make 1,000,000 ton behemoths, sure, they're either going to be made almost entirely of engines or else be excruciatingly slow. When talking about feasible designs and doctrines that work against human intelligence, there is an upper limit to reasonably effective ship sizes that's somewhere around 100k-150k ton extremes, give or take depending on the doctrines involved. Again, this is due to engine maximum size being 2,500 tons and engine redundancy giving diminishing returns. That 20th engine hardly adds anything of value to your ship, yet costs an extra 2,500 tons of space. Of course there are always exceptions, but they're exceptions and not rules for a reason. That 1,000,000 ton behemoth might actually be a serviceable jump point defense ship if it's brimming with the right weaponry, since speed doesn't matter so much on jump point defense.
This is what makes discussions of modern naval roles in Aurora worthwhile. Especially in terms of making it easier to help new players understand ship design and fleet doctrine. No one is saying you have to play a bog standard fleet that mirrors IRL fleets. What we (I?) are saying is that it's worthwhile to consider them and incorporate some standard terminology from real life in order to avoid confusion among ourselves when talking about these things, especially with newer players or when making a guide.
Also, regarding cruisers, they're called cruisers because they cruise. From Dutch kruisen, "to cross, sail to and fro." Back when a ship's travel range was a necessary consideration, the cruiser classification made a lot of sense. These days, any respectable warship can cross oceans and so the classification has become somewhat obsolete. In the vast majority of aurora games, however, range is a primary concern of ship design - and so the cruiser classification sees a lot of prominence and use. The cruiser classification doesn't really have anything to do with "independent operations" (that's actually more the purview of capital ships, give or take, see the wikipedia article I linked earlier). If a ship has the range to "cross oceans", it's a cruiser. In Aurora terms, this means that a ship that is capable of traveling through many systems without refueling would be, by description, a type of cruiser. They're the sort of ship that can go from one end of a massive colonial empire to the other without needing to stop for fuel. They don't have to have weapons - you could have all sorts of support cruisers. You could have a point defense cruiser. Any type of <adjective> cruiser is a possibility!
It's also just common sense that unless a word has an agreed upon meaning, the word is pretty much useless as a means of communication. There's nothing stopping us from making ships with no hangars and then calling them carriers except the need to have other human beings understand us. I could make a ship with no engines or fuel and call it a cruiser, even though by common understanding of the word it isn't one. The same could be said of any words. I could call my dog a cat.