I don't know if this question was ever answered but I am relatively new to large format printing and running Onyx 11 and ran into this same problem. Nearly every file I was ripping would get resized to letter
paper size and I couldn't figure out why. I finally found enough info to get me in the right direction and I fixed the problem.
In the Quick Sets there is an option for resizing. It was checked to change width to 8.5". I checked Source instead and get exactly what I open in Onyx now.
Just thought I'd answer the question, even though it is 5 years old, in case someone else runs into this problem and can't figure out how to fix it.