Very interesting about the height. Was he measured with a measuring stick on a flat, level surface with no shoes/with the same thickness shoes both times you measured him?
As you're horse must be very strange to have grown almost a hand as a 9-10 year old. The larger rug can be easily explained by the increase in condition, as the horse gets broader the rug is going to 'shorten' in comparison. My new boy is only 16.1h and quite compact length wise, but wears a 6'6 rug where as my last 16.1h tb who was much leaner than Hugo wore a 5'9 rug. Weight does make a lot of difference in the fit of a rug.
Also as Kassierae said, he hasn't changed in conformation, simply in condition. Once a horse matures at around 6 depending on the breed, their bone structure does not change. That's like saying a 30 year old adult who is 5'4 suddenly sprouts up to 5'6 in a year and suddenly develops super long legs and smaller feet. It just doesn't happen. They can develop muscle though to the point where they appear to have 'changed conformation'. Just as a weedy little teenage boy can go out and be a body builder.
In saying that, he has improved beautifully in condition. I am not sure what the intention of this thread is though, do you want us to critique his conformation (this is a critique thread) or just to pat your back and say well done you fed your horse?