women are not some special species above criticism
if you are with them and them being attractive is an important component of the relationship and they start getting fat, yeah you should tell them. it's better than dumping them and lying about it or just sticking in the relationship because you'd feel guilty.
most women are going to say something if you lose your job or gain weight, you should do the same.
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http://www.youtube.com/watch?v=npHDxSvwCE0