You'd be crazy to pay more than a few cents more than the market price for 1 share of AAPL, because that price represents the value the most marginal AAPL investor places on the stock: Offer him a penny more and he'll sell it to you. But there are investors standing behind him who think AAPL is worth more than the current price. As you need to buy out less marginal investors, the price to get them to hand over their AAPL shares will rise. Thus 1,000,000 shares will cost you more on average than one regardless of the value of control.
The same works in reverse with AirBNB: If they sell one share, it will go to the most enthusiastic investor who thinks they're a future $1T company. If they try to sell 1,000,000, they have to work their way down into those investors who believe AirBNB is a more marginal opportunity, and the average price will decline.