Well, let's look at it from this point of view:
Generation 1 Prime transformed into a truck, and as such had very boxy components. Ultra Magnus transformed into a bigger truck, and had even chunkier components to compensate. Bumblebee became a Bug, and was very small. The larger the vehicle, the chunkier the robot had to be to compensate.
This is shown in Prime, as well. Arcee was the smallest of the team, and transformed into the smallest vehicle, while Bulkhead had largest mass and had to transform to a much beefier vehicle. In robot mode, parts of the vehicle mode were visible, and this would change when a new bot got a new form. Optimus acquired different components in his robot mode once he became able to transform into what was effectively a semi cab from Mad Max. Even if he already had a large robot mode, his body had to adapt to a new vehicle.
Soundwave transformed in a somewhat Cybertronian military drone, which required a small amount of mass. If he scanned a truck, would he just be able to materialize mass out of nowhere, or would his body have to change to adapt the new size requirement for the mode?