2 of 3 NA datacenters have next to no raiders in general. That's a different issue though.
I've really never noticed any instances where Healers outnumber Tanks, especially in the current raid tier. It's been mitigated a bit in recent weeks though. It feels a little more balanced than week 1. If I had to guess, it's that everyone is just gearing alt classes, and don't want to raid week 1 on Healer because of the general trend of blaming the healer for anything going wrong in prog.