Just the Right Size: Why Big Animals are Big and Little Animals are Little (2009) by Nicola Davies, illustrated by Neal Layton.

Entertainment for kids is full of scientifically impossible ideas: superheros who can lift buses, climb walls, fly through the air and fight giant spiders or other creepy crawlies.

Nicola Davies sets them right with this book and the BTLT (Big Thing Little Thing) rule: “If you DOUBLE the length of something, its surface area and cross section go up FOUR times, while its volume and weight go up EIGHT times.”

For the very teeniest fliers, such as insects as small as one of these letters, takeoff is easy. A puff of wind on their wings is enough to get them airborne. But because of the BTLT rule, flying gets harder the bigger you get.

If you could take an insect and make it twice as big, its outside (or surface area) would get four times bigger, which means its wings would also get four times bigger. Its muscles, too, would be four times thicker and so four times as strong… This would be fine if the insect were just four times heavier, but because of the BTLT rule, it would weigh EIGHT times more. So it wouldn’t be able to take off unless its wings and muscles were much, much bigger.

Each page takes on a different facet of the BTLT rule. There are illustrations of how big our feet would have to be to skate on the water like a water strider (really big) or how big and hairy our toes would have to be to walk on walls like a gecko (like basketballs). There are pages on why a blue whale is probably the biggest animal there will ever be and how lungs with bigger and bigger surface areas for getting oxygen were the key for reptiles, birds and mammals to get bigger than a bug. 

Just the Right Size is so full of interesting science I hardly want to admit to my one quibble. It’s the size of the book. At just 6 x 9.5 inches, it’s too small for the visuals to pack in as much information as the text. That aspect seems like a lost opportunity with a book as good as this.