Why do they have to estimate it? Aren't they able to check exact size of their database(s)?

The author does not work for Google.

>Timothy has been using Google Earth since 2004 when it was still called Keyhole before it was renamed Google Earth in 2005 and has been a huge fan ever since. He is a programmer working for Red Wing Aerobatx and lives in Cape Town, South Africa.

I'm sorry to say I laughed out loud after reading your comment, but I actually wondered about the same thing while reading the article. :)

You have to admit it would be hilarious if they didn't know the size of the database or wouldn't be able to calculate it easily.

The author of the article does not work at Google. So the article is basically speculation. He has NO CLUE how Google stores the image data.

Surely he could just email the Google engineers and they'll tell him.

It's not the case here but it's actually possible that in some cases it's just an estimate ( MYSQL InnoDB ).