You know your ping list is too broad when people start dropping out in large numbers. I seriously try not to be obnoxious about pings. Originally, I thought I "knew" who should be on my list, but I made mistakes, and I learned not to make those decisions for people. For the last 2 or 3 years, I only add people who specifically request to be included. And even then, I check out their postings to be sure they're right for the list. I guess I take this stuff too seriously.
No, I think you take it just right. My lists, with the exception of the short list which is my personal list, were all created by request only. I don't get people dropping in large numbers but do get the occasional reqest to be pulled. Everyone was warned it was high volume. The list seems to rest consistantly at about 200 names. The GGG list is also about that size. It's not one of the larger lists on this forum but propably contains some of the most dedicated as all are activists in these areas.