I have the exact same problem with my zebra danios. at first i was alarmed to see the biggest one that i call "bertha" chasing arround the littler fish but after observing this behavior for some time i've come to the conclusion that it's completly harmless. the big danio might chase the little ones a bit, but it never actualy bites them or hurts them. also it happened all the time when i first put the fish in together but know happens less and less frequently so it could be a matter of one fish just establishing itself as the dominant one in a new environment.
you have 11 fish, how big is the tank you have for them? you need at least a 10 gallon tank for the fish you have while they are young and 20 gallons while they are adults. they might be acting more agressive because they dont have enough space maybe?