This is pretty typical older sibling behavior. A few bits of advice: do not use "bigger" as a reason for adults being in charge..... Say that you are the parent or grown up, but don't say "bigger" or your child will assume that with "bigger" comes authority. Older siblings often assume that they are in charge of the other kids because they are the biggest. It is very important to stress that although you love your children equally, and they have equal importance and rights in the home, sometimes one child needs more attention, and sometimes another one does. Make sure that you are clear on what his responsibilities and privileges as "big brother" are, or he will make his own assumptions. If you put him "in charge", like watching his little brother while you are in the bathroom, or on the phone, make sure that you thank him and relieve him of the duty when you are back. If you give him any responsibility for things, like telling you if there are problems with the younger brother, make sure that you thank him, and not get annoyed with the "tattling" that is ineveitable.
If you have done this at home, when you are in groups with other children, you can just remind your child, in front of the other parents, what your expectations are:
something along the lines of " remember that you are not in charge of the other children, but if you see something that you think is bad, you should come and tell us." The difference between tattling and telling is tough for children, but this discussion needs to be had over and over again. It is tattling if you just mean to get someone in trouble or to make yourself look good, and it is telling if someone is doing something dangerous or is breaking the rules.Finding the balance point is tough..... best of luck, L.