The term zerging is used often by a lot of people to mean different things. It would be nice to have a shared definition of the term, so my proposal is: Zerg: A zerg is any set of people who run around in a group, organized or not, who mindlessly spam arbitrary skills. The mindless spamming of area skills, be they healing, damage, or cc are known to cause lag.
The term comes from the game Starcraft where the Zerg race of creatures was known for running giant groups of less-powerful group members and taking down opponents by sheer force of number (and not necessarily by skill or technique). By this definition you could have a small group zerging, or a giant group (such as multiple groups of 24) zerging as long as the opponents of the zerg are lesser in number than the zerg.
That raises the issue of what do you consider a small group of 8 people who have randoms (I use the term randoms instead of PUGs, as by definition PUG is a group) following them around? I have often run groups of 8-10 where less than one minute after starting something, there are over a dozen randoms who show up – or another organized group who show up! It would not be reasonable to require that every time other people show up that the group move away, as they would spend all their time running away from their own faction. Thus, you can be part of a zerg even if you or your group is not voluntarily zerging.
If someone calls you a member of the zerg, this is what they probably mean. As you progress in skill and technique, you’ll find that you probably prefer running in smaller groups where individual skill makes a difference, and will only organise or participate in large organised groups of groups when it’s absolutely required to fight a zerg from an opposing faction. Nobody likes zergs (with some exceptions of people who don’t know that it’s possible to be successful outside of one), though the game in its current state often requires that a very large group be used to fight another very large group.