I've always changed mine earlier than a year too.
With Phoebie she had puppy complete food till adult teeth through - changed onto Junior then as I felt it was better for her to have bigger kibble. Used one or two sacks of Junior then I'd guess by about 9 months she was on adult.
Part of my reason for changing was she was getting choosy about her food so seemed as well to change her onto the adult Sam was on as she was more interested in his grub

Made it easier for me that way too instead of 2 types of food needed.
She was the latest change over I'd done with any dog I've had (only 4). With the others they were on adult at about 6 months. I give them high quality food as a pup for 6 months or so to give them a good start (including milk) and then ease off on protein levels.
I'm not sure why but somewhere along the line I'd picked up it was better to have
steady growth than a mad spurt as a pup that the body could barely keep up with. This may be a load of rubbish but my dogs have looked like John said somewhere in another thread - looks like a puppy when they are one, looks like a junior when they are one - instead of little (or not so little) adults. Just today someone commented to me that Phoebie being a slower to mature bitch isn't a bad thing in the showing world - they tend to stay at the top longer once they get there than some pups who peak early but don't seem to stay with the places as an adult. I'm sure this isn't alway the case though but has been said to me by a number of people.
As adults all of mine have been full size - changing food early hasn't affected them reaching height/size but they got there steadily by about 18 months and not at 6 months.
I can't give proper facts Lel for why I do or even if I'm right - it has worked for me though.
SaraW