I checked out the inside of the Volvo 670 and 780 for the first time

The 670s are certainly smaller. Little differences. For example, pay attention when you are close to a 780. Look at the outside of the sleeper. You will notice the side of the sleeper is not flush with the cab. The 670, 630, etc is. That turns into the sleeper-area being wider in the 780 than the 670. That converts to cabinets under the bottom window on the 780 and none on the 670. The table in the 780 is also larger and more stable and the upstairs bedroom is larger (width as well as depth). That upper bunk also has a mattress and the 670 is not much more than a hammock. The refrigerator is also larger in the 780 as compared to the 670. The cabinets are larger in the 780.

I don't think there are any differences from the seats forward.

The only reason my boss finally downsized to the 670, much to our dismay, was because they weigh 2000 lbs lighter.

These comparisons may be different depending on how the trucks themselves are spec'd but those were the differences in the Volvo 780's and 670's that my boss had.