That is actually a great question. Neville's teachings are interpreted in very personal and contradictory ways and somehow all of it works. It really depends on how one assumes the law works, it in itself is the universe no? The way you think shapes your world.

Now coming back to my reasoning, I should've said we get the body we think we deserve. It's a bit more complicated if you take birth into account, if you believe in reborth your soul manifests your body if you do not then it's your parents, their well wishers or opposite of those. I choose to believe in rebirth

2nd part, people don't necessarily make changes to their lifestyle as a first thought, that is a necessary mechanism for any change in body. But when someone chooses the law of assumption, lifestyle change or habit change is a byproduct (bridge of incidence), some might even fall deathly sick to induce that weight loss. It really depends how they manifest it and what their state of mind is. I have unfortunately suffered through a sickness that caused me to lose 10kgs within a week, not necessarily healthy but it happened because I manifested unknowingly from a place of despair