If it isn't fatphobia, then it isn't what the op wanted to start a bunfight discussion about.
I personally think there is a risk you may make things worse and I think that you "saying something" is highly unlikely to be a magic, weight loss, wand.
However, if you honestly think this person has no idea their health is suffering due to their weight, and you think you can raise it without pushing her into comfort eating, hiding her eating, (which is a disordered way of eating), causing her a lot of stress or making her want to hide away altogether, then go for it.
If not, then you need to ask yourself why you feel the need to tell her. What are you really hoping to achieve?
Spouses are tied together legally in a way that adult relatives are not, so it is not a given that it is your place to say anything at all.