It isn't the size of the load in most cases.
I had Aiki Trucked from Virginia to San Francisco for $9,500.00. The trick to getting cheaper shipping is to tell the trucking company that they can do it whenever they want to. Get the boat ready, have it ready, and then they can pick it up on a return trip and save you money as they have to come back anyway.
I would call the guys at Anderson Trucking in Canada. They were the best priced that I have found and did a great job! Including documenting that the yard in Virginia was trying to rip me off... Something that was above and beyond what they had to do, convinced me of their ethically business model though. I did let them do it on THEIR schedule, which changed about 5 times before the boat got here, but in the end I was a completely satisified customer and they beat everyones prices. When I asked about insurance they asked if the boat were valued at over a million dollars. When I said no, they said that their standard insurance was for a million, and that I didn't need to pay anything extra for it. They also said that unlike other truckers their insurance was PRIMARY insurance, and paid first, not after you tried to fight it out with your insurance company. ( I have a horror story about a Lord Nelson tug that hit a low over pass and secondary trucking insurance.)
I have also worked with Dudley which does a great job if you can get them to call you back..... Having to hound someone that you want to drop that kind of cash with seems strange to me.
And Joule trucking did a good job with my dads boat on the east coast.
Note that there are a lot of peope out there that offer yacht transport, but don't actually have any trucks. They also don't have insurance or a transport liscense. They give you a low ball quote and then try and find someone to transport it on the way back for the low price that they quote. The issues are that should something happen to your boat on the way, no one has any insurance that is responsable for it.
Guy