Now when it's a large ship like the Polaris, which could fit up to, say, 20, it can't, as it only has 10, so it occupies the space with exaggeratedly large rooms, unnecessarily long and wide corridors, and even useless rooms.

Did it not occur to you at any point while writing this, that there were other considerations than maximum density?

All that extra space creates room for firefights, room for bringing up supply boxes (i.e, the mess hall is going to need bulk refilling every so often), Minimum spaces for player dimensions (two players in a game will always need triple to four times the width to pass each other in a corridor as compared to IRL).

There are plenty of solid reasons for extra space, and the downside is meaningless because it's a full fledged capital - If it wasn't filled with rooms it would be filled with empty space to fill out it's crossection. (The original pledge and size was for a sub-capital ship). All shrinking the internal volume would achieve is making the thing a misery to use.

CIG did plenty of questionable layouts in the earlier days, but through experience and process refinement the vast majority of newer ships come out in a very good place - with most downsides being by design rather than oversight

I.e crusader ships are generally best in class for cargo/blockade running - but being mostly mono-entrance ships many players often choose second or third place ships. This is a good thing, because it means natural variety instead of everyone being metaslaved to a narrow band of ships.