What is the difference between
It was as big around as a truck wheel.
It was as big as a truck wheel.
A good question, Dmitry! I've looked around the interwebs and found this definition:
So, by saying
we are stressing the fact that this something is round or roughly round in one of its sections.
I've met around being used in this way before, and used to imagine a person circling his arms around something to take stock of its girth.
There is a very good poem I know that applies this word in this sense, adding to the imagery:
Note that the blocks of beech could be much thinner than the chopping block, but they are as large around as it is.