Are the robots in the transformers films really transforming?
1. They HAVE to cheat
First of all, having a part that is not showing in the frame will consume render time to be computed, even if it is not showing. The files are heavier to handle and all the process becomes sluggish.
2. Transforming in a 3D program is given by 3 main operations

Moving
Rotating, which they are pretty obvious in a transformation.

But you also have

Scaling. If you have something scaled so tiny that you can not see it, the geometry is still there consuming resources of the computer that are wasted.

3. The goal is
Telling a story, a fantasy story about fictitious characters, not to be accurate on a transformation.
The other main goal is to make things look cool, somehow logic and accurate but mainly cool.
4. Autobot's universe actually has parts that &quot;disappear&quot;
Take for example the Spark humungous building-sized cube. It does not only compress in size to be handheld... It would have the density of Jupiter nuclei, weighing the same as a building. But somehow things there just went &quot;poof&quot;... So, again some liberty can be used on the transformations.