There's often a confusion between people who go "whole-food plant-based" for personal health reasons, and people who go vegan which is about animal's rights. Of course a person can want both too.

In any case, people who adopt more plant-centric diets do often see a reduction is bodyweight, but it's not guaranteed. As the pic up top shows, it can still be very easy to have an unhealthy lifestyle even if it is plant-based.