Hey Yujini. It means that you should eat the food in small portions. It would not be relevant or rather make good sense to say you should eat good food infrequently.

It means in small amounts. However, the sentence doesn't make perfect sense because usually we would say to eat fried foods, unhealthy foods, and desserts sparingly (or only a little bit). People need to eat a reasonable enough amount of healthy food and to get enough protein.

It means don't eat too much. It isn't specific about whether this should be smaller portions, or eat less frequently or both. There is a popular book called "In Defense of Food" which promotes the suggestion to "eat food, not too much, mostly plants" which gets at the same idea. In other words, eat real food (not junk food), eat sparingly, and emphasize plants (rather than animal products or flesh).