I am not sure what the problem is that you are trying to solve.
The engineer likely needs to be tall, and it makes perfect sense for it to be a tier 2 unit. It has to fabricate extremely large buildings in a very short space in time. If it was short and pudgy then it wouldn't be able to see the top of the buildings and might miss a few spots with the paint job. Then the engineer would have to answer to its supervisor, lose its job, and would not be able to help support its family.
The medic however, does not need to carry around the blueprints, permits, site inspection reports, licenses, excavation equipment and massive tanks of plasma, so it can be small and nippy. Ambulances are required in many territories to operate emergency lights while on duty, but perhaps it would also make sense to put a flashing orange light on the engineer and decrease unit movement speed near construction zones so that we can keep OSH happy.
The engineers role is simple: Build.
Whether this is factories to train units, assist factories to build units, or expand the base economy and defenses, they should always be kept busy building.
There will likely be a lot of changes to aesthetics as things progress. The game is still in early development, only one of the playable factions is available. This particular faction has found that spraying controlled bursts of plasma into a contained structural electromagnetic field where it cools and solidifies is an efficient enough method for fabrication without the need to use expensive and inefficient lasers. Perhaps the other factions will have developed technology for construction that is different, which we might get to see in the future as they are released.
I am sure if they figured out how to magically levitate all of the vehicles, they certainly would have not forgotten how to use gyro stabilizers.
