I had read that "glutton" is used for somebody who eats a lot. Is it used commonly or are there any other words that are more common?
"Glutton" is a useful word However, it is a little old-fashioned, it carries a negative connotation. It is someone who overeats (and gluttony is one of seven sins). When you want this connotation it is the right word to use.
If you don't want that effect I would just describe:
Mary eats a lot, but she exercises every day so she never gets fat.
Other words: "Foodies" are people who love good food but don't necessarily eat a lot of it. Foodie is rather casual. A "gourmand" is someone who is extremely (and often excessively) fond of eating and drinking. "Pig" is certainly insulting, but can be heard used as a joke between friends. (I suspect that the term "pig" is even more insulting in other cultures) To gorge is to eat a large amount at one time, so a gorger is a person who will stuff themselves. Again, this would be viewed negatively.
It seems that English speakers tend to view people who eat a lot in a generally negative way. The plain description "she eats a lot" seems to be the only neutral way to say this.
glutton is the commonest I've heard. Other words that I use include
pig, hog, and gourmand
Please note that the first two words could be offensive.
Another word that I can think of is a stuffer. Have a reference: It reads...
Stuffers purposely overeat to gain weight, posting before and after pictures on the Internet to chart their progress. It's scary stuff...