It is, generally, rude, and frequently extremely noticeable, when one person takes an excessive amount of food or other items that was meant for a group to share.
I had a guest at my home do it once, and although he was welcome to partake of my hospitality, he consumed approximately 75% of the food I had prepared. I had made enough food to feed about twenty people even though there were only about 6 guests and myself. We realized it was him because he'd made repeated trips back into the kitchen and had apparently been stuffing his face while standing in the kitchen.
I got a little bug-eyed but didn't say anything. The other guests, who for the most part had known him a bit longer than I had, laid into him on my behalf. It ended up being kind of an intervention for him - he learned from the experience and was much more polite about the quantities of food he took from shared dishes in the future.
This happened at a dinner to which I was invited back in my teenaged years, and even at my young age, I knew what a certain guest did was very rude. I learned from it.
I was out west visiting an aunt and uncle, who lived in the California desert, and one night, the skies opened up and poured rain, which caused a major flood in their town and neighborhood. (Some people living near them actually lost their lives, it was that horrible.) Their house was completely filled with water, which we spent the day trying to dry out/clean up as best we could.
Neighbors of theirs down the street lived on slightly higher ground, and had suffered no flood damage at all. They invited us all down for a meal and a rest later on, which was kind-hearted and very welcome.
In the midst of this chaos, a family friend, Clueless Magoo, shows up. He lived near the beach in L.A., and had just moseyed on down for a weekend visit without any notice, and without calling first. He basically sat there and *watched* us suck mud and water from the house with a shop vac and brooms and mops, all the while complaining that the pool was full of mud so he could not use it.
My aunt called the neighbors and told them there was an additional person at the house, and was told, of course, bring him, too. The neighbor wife was of Indian descent and had prepared a delicious ethnic meal, chicken and rice and some veggie dish. We sat down, and Clueless Magoo helped himself to easily *half* of the chicken and rice dish. The look on the neighbor wife's face was priceless, and one I will never forget. She was too nice a lady to say anything, but my uncle was not one to hold back emotions or thoughts, ever. He said something along the lines of," hey Clueless, what in the ehell are the rest of us supposed to eat? You weren't even supposed to be here and you are helping yourself to all the food, you ignorant so-and-so." Clueless just shrugged and said he was extra hungry from his drive to the desert, (about 2 hours, maybe 3 in heavy traffic), and scarfed down his meal. (I realize my uncle was rude, also, but I think Clueless wins the grand prize.)
There would have been plenty for everyone had Clueless not basically emptied the serving bowl onto his plate. I was very young and inexperienced in etiquette intricacies at the stage of my life, but from that night on, I always made it a point to take very little food on the first pass, so that nobody would ever have to feel as uncomfortable as we all did at the dinner table.