Chances are they already know. People who are not considered overweight or at least not that big often thing that larger people somehow just have no idea they're big, but really it's not the case. Once you go in knowing that, it really depends on the relationship you have with this person. Are they family, a close friend, a domestic partner? Best to consider the relationship before opening your mouth and blabbering on about something that's already quite apparent. It'd probably be slightly easier if you're around someone a lot (aka, family, spouse) so you can suggest something like trying to lose weight or keep in shape together. The last thing you want to do is call the person awful names. That'll just hammer the last nail into the coffin.