I'm trying to over ride a default quota using a group quota, so that some of my users get a bigger quota in their home directories than others.
User quotas and group quotas are two different things, and they are enforced separately. A specific "user" quota can override a default "user" quota, but a specific "group" quota cannot.
So what you've done is put a 50M quota on every user, including the ones in bigusers. And in addition, you have put a 100M quota on all files in the bigusers group.
I'm afraid you'll need to add a separate 100M user quota line for each person in the bigusers group.
Dave