Size Changing

Although scale has never exactly been a deep concern when it comes to Transformers, given that the series has featured F-15 jets not much larger than sports cars, it is not uncommon for certain Transformers to clearly and deliberately change size when transforming from one mode to the next. The methods employed for this startling alteration in physicality have differed from timeline to timeline, and have long been a heavily debated aspect of Cybertronian technology.

Examples
Note that many of the Transformers named here have also transformed at certain times without shrinking or enlarging, retaining an alternate mode to scale with their robot mode.

Shrinking
The most famous examples of size-changing Transformers are Megatron, who becomes a pistol, and Soundwave, who becomes a stereo cassette player. These two Decepticons shrink as they transform, assuming one of two sizes – one that allows another Transformer to wield or operate them, and another, even smaller size, to scale with humans. Other shrinking Transformers include Reflector, Blaster, Mini-Cassettes, and Perceptor.

Enlarging
Conversely, Transformers have been known to grow in size when assuming their altmodes &mdash; the most prominent example is surely Astrotrain, whose space shuttle mode could become large enough to convey a whole contingent of Decepticon warriors, and even allow Devastator to form inside him. Other Transformers that enlarge in such a manner also generally do so for the purposes of transporting others; they include Omega Supreme, Skyfire and Cosmos. Combiners are often larger in their combined form than their robot modes would otherwise indicate. In the original animated series many of the Transformers arguably enlarged or shrank in vehicle mode to a lesser extent, this is most notable among groups such as the Decepticon jets and minibots, whose altmodes could not possibly turn into robots that are the size they are depicted otherwise. Whether this was truly intended to indicate actual size changes, was merely "lazy animation", or was simply a stylistic choice designed to emphasise the awesome stature of robot-mode Cybertronians, is a matter of individual opinion.

Implicit size changing
Transformers that act as transport for other Transformers often implicitly change size to do so. However, size changing is rarely explicitly depicted in these cases. Instead, the following method is used: A character such as Astrotrain transforms in-frame from robot to shuttle with no visible expansion of size relative to his surroundings. Cut. In the following shot his fellow Decepticons are shown running into frame, suddenly relatively tiny in stature so that they can board the shuttle. Although Astrotrain has not been shown to actually grow, one has to infer that this is more likely than that all his confederates have individually shrunk.

The same change-in-scale-via-implicit-size-changing is used with Omega Supreme and other characters used as transport. For example, in the episode "Make Tracks", Hoist, who is depicted as one of the larger Autobots in robot mode, is shown riding inside Huffer, a Minibot whose truck mode is usually depicted as being smaller than Optimus Prime's. So either Huffer is extremely large in that scene, or Hoist is suddenly the size of a human.

(The other explanation is that the animators simply hoped no one would notice the sudden change in scale. Since Transformers is usually targeted at kids, there's probably some truth to this.)