In Chapter 433, Robin (although it was not shown in any panel) joined the crew's meat-eating competition and judging from the crewmember's remarks, it seems that she has a huge appetite.
I've come up with a logical explanation as to how she was able to do this despite the fact that she has a slim figure and she seemingly does not do enough activity to lose weight.
With her devil fruit powers, she can be able to "grow" other stomachs, esophaguses, intestines, etc. in order to transfer the food she eats to other places. She can also have them digest the food for her so that she keeps her figure while having an enormous capacity for food intake.
Yeah I'm bored. Please bear with me.