comparatives vs contrasts

[quizziful]

“The car is small. The truck is big.
The car is smaller than the truck. The truck is bigger than the car.”

Recently, I read these sentences. Is the comparative usage correct? I think making a contrast between a truck and a car would be the logical approach, i.e. "The car is small. The truck is big", without attempting to make a comparison. The first two sentences make it clear; the latter two sentences seem to be incorrect or at least unnecessary and confusing to a learner of English. To me, it doesn't make sense stating that "The car is smaller than the truck", given that the truck is big, and then turning it all on its head by saying, "The truck is bigger than the car". Compared to the truck, the car isn't big: "The car is small" Surely, when saying that one thing is smaller or bigger than the other, either they both have to be small for the former case or both big for the latter. Other similar 'comparisons' were made between light and fast, weak and strong, and slow and fast things.
Am I incorrect?

[Raymott]

“The car is small. The truck is big.
The car is smaller than the truck. The truck is bigger than the car.”

Recently, I read these sentences. Is the comparative usage correct? I think making a contrast between a truck and a car would be the logical approach, i.e. "The car is small. The truck is big", without attempting to make a comparison. The first two sentences make it clear; the latter two sentences seem to be incorrect or at least unnecessary and confusing to a learner of English. To me, it doesn't make sense stating that "The car is smaller than the truck", given that the truck is big, and then turning it all on its head by saying, "The truck is bigger than the car". Compared to the truck, the car isn't big: "The car is small" Surely, when saying that one thing is smaller or bigger than the other, either they both have to be small for the former case or both big for the latter. Other similar 'comparisons' were made between light and fast, weak and strong, and slow and fast things.
Am I incorrect?

Your first line isn't a comparison. Your second line is.
Here's another contrast/comparison to ponder:
"My cat is big. My horse is small."
"My horse is bigger than my cat."

As far as two things having to be big or small, no, that is not a feature of English. The terms are relative. The Earth is bigger than Venus. But both are small compared to Jupiter.
To say that something is big, we are saying that, of its type, it is a big example.
"Elephants are big" is only true if you're assuming that you mean "compared to most animals". But elephants aren't big compared to mountains.