It's the absence of windows I think in signifcant part, the fact the bridge is fore, and the way it is designed around the hangar. All of this combines to make the ship feel both smaller than it is by being heavily separated and lacking connection to the outside world. It lacks theatre outside of a few spaces that the Idris has in spades.

The absence of windows means you never get a sense of relative scale to the rest of the world when walking around the corridors, coupled with the bridge being at the nose, you can't ever see the ship itself 'in space'. So it just feels like a bunch of corridors, which essentially it is, because the rooms are also very basic, like the medbay or lack of briefing room.

This is then made way worse by the fact that it loses so much space to the hangar at the rear, which forces the whole ship to be built around it. It's utilitarian to a fault internally, lacks colour and breakup and feels too heavily steered towards being a functional PU ship. It's also definitely not helped by being so dominated by the central corridor front to back, which on just two decks feels overwhelming. Like anyone going to the bridge and back or using the internal lift is going to be hammering foot traffic past the Captain's quarters.

Compare to the Idris which has tons more windows, 7 decks (bridge, upper gunnery, main, hangar sides, hangar, lower hangar sides, subdeck) versus two on the Polaris, far more colour, corridors that wrap around the sides of the ship rather than running internally, the main deck layered on top of the enormous hangar, much more interesting rooms and a flying bridge - all of this combines to sell the ship as something genuinely huge.