The world is 30 million blocks across. A block is one square meter.
30,000,000 meters is 30,000 km. 30,000*30.000 is 900,000,000, so the Minecraft world is nine hundred million square kilometers.
The earth has a surface area of about 510 million square kilometers.
It IS larger than the earth, people!
(However, since it’s only 128 blocks deep at the moment, it’s still significantly smaller in volume)