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, his and Soundwave’s cassettes and Perceptor.

Enlarging
Conversely, Transformers have been known to grow in size when assuming their altmodes – 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 and Cosmos.

Explanations
Although size-changing has been evident in Transformers from the very beginning, the original animated series and comic books never address the topic, and it was regarding as simply being the way of things. Fandom debate on the topic has seemingly inspired later fiction to offer varying explanations on how it functions.

Dreamwave
Dreamwave offered two explanations for size-changing within its continuity.

The first, to explain enlarging, was parts compression – the notion that the Transformers in question, such as Astrotrain and Broadside, had many dense layers of armor in robot mode, which then slid out, expanding to create a much larger, but less-strongly-armored vehicle mode.

The second explanation, to justify shrinking, was mass conversion. This method put it that Transformers like Soundwave were genetically endowed with the ability to transform not simply their bodies, but their entire molecular structure. This ability, not under their conscious control, only activates when they transform, shifting their atoms according to a pre-determined schematic.

IDW


IDW's Escalation sees Megatron employ a mass-displacement sequence, physically discarding a portion of his bodily mass to allow himself to shrink to a human-scaled pistol of appropriate weight and density. What with E equalling MC2, the loss of this amount of mass (shunted to an undisclosed location in space and time) results in a volatile energy discharge, requiring bystanders to step away lest they be injured by the forces released.

Fandom explanations
The issue of shrinking and enlarging was, in the early days of the online Transformers fandom, a frequently debated subject. The Transformers were clearly displacing their mass, to allow their shrunken forms to be operable – so where was that mass going? The postulated answer to his question was subspace, the void of sub-dimensional space between realities.

This explanation also served as an answer to the top Transformers question of all time ever – “Where does Optimus Prime’s trailer go when he transforms?” The answer? Subspace. Furthermore, in a few sparse instances throughout the American animated series, and with alarming frequency in the Japanese shows, Transformers’ weapons can also be seen appearing and vanishing in a flash of energy. Where are they going? Subspace. You get the idea, folks.