Weight And Body Structure

Have you noticed the player progression chart that appears everytime you start a new season in your dynasty? Well my question is basically this:
Does the weight and body structure change randomly or not?

I've been using the Cavs.. And Lebron used to be a medium and cut player... and when I started the new season he turned into a heavy and uncut player while gaining a lot more weight...

Is there a way to keep players in shape? Do I have to train them a lot or stuff like that?

Thanks in advance...

Well, there is this guy who used to gain a lot of weight and became heavy and uncut... but when I starting training him more on athletics, he became medium and cut again.. but I'm not sure if it's just a coincidence..

Ok thanks.. I guess it helps somehow...