MQ, as I said, both DH and I have both put on about 2 stone in the last 5 years. I think we would both agree that we would find the other more attractive if we lost a bit of weight, but actually we like our lifestyle a bit too much to do it.
However, if I were to want someone to encourage me to lose weight (even if they were thinner than me), I think boosting their self-esteem is the greatest idea, if they feel better about themselves they are more likely to be encouraged to make changes.
For example, DH bought me some lovely (and well fitting) underwear for my birthday and, since then, I have made far more effort with me hair removal, exfoliating, general declothed self (sorry, that maybe too much information 
.
Likewise, I tend to eat biscuits and crap if I'm tired or run down.
Therefore, if you want her to make changes to her lifestyle I suggest you do it yourself! Cook her some delicious, low calorie food, suggest you both go on walks, bike rides at the weekend.
Spa weekend maybe? You can see where I'm going with this can't you .
In short, if you feel that she is no longer making an effort to look attractive and you feel that is part of a wider complacency about your relationship (I feel this about DH shitting with the door open) then that is one thing, but if she is attractive and puts effort into your relationship, but just happens to be bigger than she was - then yes, you are being shallow.
I have to say, I do really feel for women (and men) who feel their appearance is their best asset ..... because it will always fade.
I have never been a size 10/12 (well maybe as a child) so I am confident that DH is not the type of man to be hung up on how much I weigh. Not that he doesn't care what I look like, just that he doesn't see weight as being the defining factor.