One way to hold this phrase: picture money moving in very large amounts. When the amount gets that big, you can stand at either end of it. Stand where the money is collected and you see the companies, big enough as a group to move prices and policy. Stand where the money is spent and you see the thing people are buying — dieting, bubble tea, running shoes — which is why an activity can be big business too. Same flow of money, two places to stand.
One branch
大錢流動的兩頭
兩個意思共用同一個畫面，差別只在你站在錢的哪一頭看。語法上也有跡可循：講公司群體時前面不加冠詞，講活動時也不加，所以 a big business（一家大公司）一出現，就是另一回事了。
noun · B2
Very large companies thought of together as one powerful group — a group with enough money to affect prices, jobs, and the decisions a government makes.
大企業（整體）