Why the big difference? As an example, let's say you were doing a Business System Design of our solar system. Since the sun comprises about 99.8% of all the matter in the system, it would probably say, "All we need is a ball of super-hot plasma about 700,000 miles in diameter. There's nothing near it, the closest object is 24 trillion miles away. Oh, wait a minute, there are some insignificant little dust motes floating around; I think one of them is called Jupiter." The Detail Design would not only have to expand the dust motes into nine planets, a few hundred moons, and a few hundred thousand asteroids and comets, but it might also discover life on the third planet, an entirely new aspect to the problem.
The Business System Design and the Detail Design are also different because they have different functions and are intended for different readers. The purpose of the Business System Design is to assist with a go, no-go decision for the project. It's a management document. Sometimes, as with this project, it happens after the decision has already been made, in which case it acts as a sanity check to see whether the wild-eyed estimates made to the budget committee are anywhere close to reality.
The Detail Design is a programmer's document. Its primary function is to give the programmers enough information to code their programs. It does catch problems that were missed in the Budget Committee presentation and the Business System Design, but the sad truth is that by this time the project is funded and underway, and the solutions to any new problems are added into the system but not into the budget nor into the timeline (Why All Data Processing Projects Are Late and Over Budget). The project is then already on its way to being late.
"God is in the details," goes the phrase. I disagree. I'd say that God is a big picture man and if there's anyone lurking in that tangled mass of details, it's the devil. The high-level Business System Design looks at the system and what it sees is a clean, orderly process with only a few parts that all interact like a perfect, frictionless machine. The Detail Design looks at the same system at a lower level and it looks more like the aftermath of a major battle during World War I, a muddy field strewn with dead bodies, smashed fortifications, and maybe a lost picture of a wife or girl friend blowing over the trenches.