Putting the HSI in the center gets in the way of a flight director, flight path indicator, heading numbers and bugs on the horizon, synthetic vision, and anything else we try and put on the screen. Also, we only have space in the middle of the screen when it's 100%, and lots of customers run 50% or 40% PFDs, so it's nice to not move things around when you change size.
There is a chance we'll change anything that customers ask us to. Changing the size of the HSI on the 7" screen isn't currently on the list but it will be if we get enough interest. Everything we do in terms of sizing and locating things is always a tradeoff between customers asking us to put larger things on the screen vs. putting 142 more things on the screen