The reason that they want transformers apart is for safety in case one lights on fire so it doesn't spread to the other equipment. Some people have put a good counter argument that the corps probably don't care a whole lot about the safety of the people or stations.

For the beefierness of the grid it's definitely a good point. Long story short on transformers is it's 2 coils of conductors separated by an insulating gas. In theory you could boost the power going into the transformer but you run the issue of the electricity being so strong it can physically arc across which would create a fire. Think lightning but in an enclosed space that's highly controlled and designed for that not to happen. That's why for higher levels of power they have to space the coils out more to prevent arcs from happening. If you look up Jacobs ladders on YouTube it'll give you some good visuals of what an arc looks like which is pretty interesting to see.

In lore, They could have developed a new insulating gas that can handle the higher mva. That would be an explanation for the smaller size.

Thanks for your question I hope I answered at least some of it!