Given that it makes the chest of the predominantly male soldiers seem wide and broad, perhaps it intends to symbolise strength and vitality to the populace? After all, the military in Cardassia is also the government, the police force, the emergency supply system, etc. Just as the Cardassian neck is broad and muscular, perhaps Cardassians instinctively associate a broad chest with a dominant male, and the armour is designed to reflect that? Authority, leadership, attractive vitality? Even a sense of inclusion for the lower-status males who inevitably form the majority of an army - "no-one is left behind in the Cardassian Union - all men are respected and admired for their service!"