Obviously, the sheer size of these big trucks makes them more dangerous, and the fact that a cyclist or pedestrian is more likely to go under them when struck because of their height, but there's another factor at play that makes them particularly dangerous to cyclists: the blind spot.
Because of their size, height and the lack of usable windows, large trucks have sizeable blind spots, particularly on the non-driver side (AKA, the side which cyclists are most likely to be on in a normal traffic pattern). In the U.S., that means the right side of the vehicle, but to get an idea of just how large this blind spot is, check out this "mirror image" video from the U.K.
I spent some time driving a contractor van like this one for a job I had not too long ago, and I can tell you the right side blind spot on one of those is significant enough that you can't easily see a large car that's riding along the right side of the van, let alone a cyclist. This can be offset with mirrors to some degree, but it's still very hard to spot anything immediately to your right very quickly.
A regular passenger car has, or should have, a pretty clear view of their right side because of the passenger side windows, but with many vans and commercial trucks, this space is blocked by cargo or the design of the truck.
This doesn't excuse unsafe driving by those who operate these vehicles, especially since as professional drivers they should be operating at a higher standard than the average person behind the wheel, but it is important, as vulnerable road users, to be aware of where a vehicle's blinds spots may be. Since many bicycle commuters may not have driven a large truck, let alone a tractor-trailer, we might not have thought about this sort of thing, and could potentially put ourselves in a bad spot in regards to a turning truck.
Furthermore, especially in the case of rental trucks (Uhaul and Penske box trucks for example), we can't always count on the driver of the truck to have the experience or training to safely deal with the large blind spots on a truck. That guy in the 17-foot Uhual may have never driven anything bigger than a Honda Civic up to an hour ago, and suddenly he's on the road with you and your bicycle.
Be safe out there!