PvP scenario, you need the firepower to get a high bounty player so you ride out with the boys in your killers. PvE scenario, in a big trader the credits per hour is just too good, and the economy of scale makes sense, also since you can trade in a cheap mostly Non engineered ship, the economy of scale works out. Or you’re doing bounty hunting assassin missions, etc.
Your ships value, generally speaking, scales with the purpose its setup for. A trade, or exploration configuration being much cheaper than a full combat one too.
Big ships scale now. That's the reason people use them. They just don't scale enough to make them worth 500mill to 1bill per loss. Because nothing actually justifies that cost structure other than Borann.
PvP especially is a lossy perspective.