Haha, you and I have both been troubled by the same problem. At first, I was surprised, too. I've asked adults countless times, and they always say I'm stupid at reading.

Let me tell you something. Doubling the population is actually twice as big (that is to say, omitting the word "big"), just like twice being four and twice being six. This is their speaking habit, and we don't have to define it by the mathematics we have learned. We should understand it and get used to it!

I see. Have fun!