Is this just a result of increasingly thinner seats to pack more in?
I remember when I got on that Hull Trains loco hauled Mark 3 formation from aaages ago, the seats were ridiculously comfortable (haven't felt seats like that since the Mallard upgrade, brought back memories of the IC Swallow era).
I don't think you can put it all down to that. The 158's, for example pack 'em in fairly tightly, yet the ex TPE ones have some very comfortable cushioning.