As a child, I was taught by my mother that asking people what they had eaten was rude. I think she was right.

It would be okay in some contexts. Like, if a coworker said to me, "I went to a great restaurant last night." then I might say, "Oh yeah, what did you have?" That, to my mind, would be perfectly fine. The other person initiated a conversation about having a meal.

The scenario you are describing definitely sounds rude to me. Also, even if fat-shaming was not the intent, this person was being insensitive. It would be like singling out a person with bad skin and saying, "What kind of soap do you use?"