There are quite a few classifications of trucks. In the U.S.:
Class 1: 0 - 6000 lbs
Class 2: 6,001 - 10,000 lbs
Class 3: 10,001 - 14,000 lbs
Class 4: 14,001 - 16,000 lbs
Class 5: 16,001 - 19,500 lbs
Class 6: 19,501 - 26,000 lbs
Class 7: 26,001 - 33,000 lbs
Class 8: Over 33,000 lbs
Classes 1 through 2 are considered “light” truck, 3/ through 6 is “medium,” and 7 and 8 are “heavy.”
Classes 7 and 8 require a commercial driver’s license.
Generally, Class 3 starts to have 4 wheels on the back axle, and Class 6 generally starts having multiple axles on the back. At a certain point, you’re up to 18 wheels on a tractor and trailer.
OP’s picture is probably of a Class 2 truck, while you’re thinking of Class 1 trucks.