How is a 20bn company going to issue 27bn worth of stock? Or are they just going to pretend the newly issued shares are valued the same per share as existing stock is right now?
In practice the price paid for the company being acquired is usually a bit higher than the market value (so the shareholders take the deal), and the market usually punishes the acquirer a bit and the resulting entity’s stock will fall a bit. (This is most definitely not investing advice.)