I don't know that this is true. An air/raft chassis for instance is the largest assembled part, but the lifters, fusion plant and electronic controls are all more complex even though they are less volume.
I am suggesting that there are 2 independent elements of time for a Maker, volume and complexity, and these are getting conflated in your example.
For a discrete part, as aramis correctly pointed out up thread, really the only thing that matters to Maker time is volume. There is no complexity per se, only precision, and that just isn't a significant time adder. Either the Maker can hit that precision or it can't. So if we are talking about a single, discrete size 1 part or size 3 part, the time taken will not be 3 times the first for the later, but orders of magnitude longer as it scales with volume.
Now in your example, you are comparing parts with different complexity - bulk chassis versus high tech components, so the relative time to make these is not just a function of volume any more, it is a function of volume and complexity.
How to handle this without going rule crazy? You already pointed the way; make time a function of cost. You could mitigate that some if the party has certain sub-assemblies available in stores (the computer chips MM writes about) so that those highly complex technologies don't have to be built from scratch, then the only thing to deal with is volume again. You could mitigate it more with high-end or large-size Makers versus ship board workshops. You could allow molds to be built for very large items, so you only have to have the Maker build the mold and that scales with surface area (squared) instead of the volume (cubed). Put as many elements in as you want, but for game play purposes, if we are looking for a simple rule, which was Reban's idea with using the size codes, I think cost is probably the way to go and just let that capture volume and complexity in one go.
(For MTU, I think gravitic, jump, high-end electronic components have to come from stores and can't just be made from scratch with a Maker. YMMV.)