it's a bit difficult but once you get your head around it it's fine

think of it this way:

the cossie spring has a higher poundage, but it's shorter, so the initial weight of the engine, which is much more, will compress the spring MORE and therefore your car will be much lower than it should be

the spring will also be designed to have x amount of load on it for cornering etc so it will compress at a certain rate, but you've already got too much weight on it so it won't handle properly either

imagine you go shopping and your plastic bag will carry one bag of potatoes, but you put in 2 bags of potatoes, the bag isn't designed for that sort of load so will be more likely to split