Ive always liked to consider cybertronians with more realistic functions and biology, so the concept of mass shifting always felt out of place to me. I never liked them just being able to shift drastically in size so I've come up with my own concept of mass shifting that follows a more biologically possible route.
So my version of "mass shifting" is essentially this:
If a cybertronian wants to be 'smaller' their endoskeleton tightens down and anything spacious squeezes down to connect, kind of like a spring when you press on it, or like a flat slinky. Then the exoskeleton (aka the outer plating) will flatten down and interlock.
If they want to be 'bigger' its essentially the opposite. The endoskeleton separates and the exoskeleton plating flares out.
This gives them somewhat ability to get a little smaller or a little bigger.