I think the guy in the truck was just telling him he was bigger. I got that feeling last week when I drove the entire length of the Pennsylvania turnpike. After a while, I was so happy to see the trucks parked on the side of the road or in the rest stops for the night!
Oooh! It was more than just that. The truck was as relentless and deliberate as could be; too much so for it to just be something casual like bullying. Serious truck drivers are always in a hurry either going somewhere or returning. I got the sense that there was a whole lot more to the story than we were being told, maybe something that the character Weaver played had forgotten from his past.