That's exactly what it is...I work for one of express companies..It's dimensional weight...

That's the reason frames and wheelsets cost more to ship..It's not the weight, it's the size..

A rough guide to calculate dim weight is to take the length X width x height divided by 166..If that number is greater than the actual weight, that will be the chargeble weight

In your case, approx wheelset box is 28 x 28 x 10 /166=47....