Problem is, the stronger you already are, the slower you become even stronger than that. I have a soldier with strength 57 at 25 missions. The lower mission count is because of strength training. A soldier gets more if he were burdened on average over the course of a mission, but I surely limit this to certain safe missions, or the end of missions, when the situation is safe enough. Not like start every mission fully packed with grenade launcher ammo. That is most strength gain then, but I can't do much with such a soldier.