I'm not a practicing pagan, so please take this with a grain of salt. But, if someone said that to me I would reply, politely:
"Yes! The Goddess made me beautiful. She also wants me to keep myself healthy and love my body, and that means limiting certain foods sometimes."
If you attend these events frequently enough, you may want to do what some other chicks around the boards do in these situations. They don't mention that they're watching what they eat... instead they say these things.
"No, thank you. I don't want any now."
"No, thank you, I'm very full."
"No, thank you, I'm not hungry for that at the moment."
"No, thank you, but it looks delicious!"
"No, thank you.
And finally, if someone keeps saying "come on come on... just a little!" one person said they replied this way, "Why do you want me to have that so much?" It puts the spotlight on the person offering.
These replies are polite & tend to work well in any social situation, religious or otherwise.