This one always works for me, but maybe it's because i'm a fat guy. It's worked on women easily three times my size though.
I'll say some shit like "Damn, these (unhealthy food), are so good but they are so bad for me"
they'll usually reply something in agreement
Then i'll say "I don't even care though, they are so good. It's been so hard to lose weight anyway"
Then they will usually say how it's been hard for them too.
Then I say some anecdote about how i've been trying for a few years and they will say something similar.
Then I will say yeah, i've gone up from...x weight... to whatever weight I am now. And then look like i'm thinking hard about something confusing and say "...how much do you weigh?" as if it's an important piece of the puzzle
Added bonus they'll probably start talking about their own weight gain all on their own after and tell you some hot story about their struggles