Certain perks are simply too weak too handle big Zeds, also big Zeds have way to much Mother Trucking Health.
I'm not sure which way you're going with this so I'm going to take this as against my personal opinion so I can give everyone my opinion. No offense was taken and none is meant to be given.
Yes, some perks are too weak to handle big zeds. That's why their job is not to handle big zeds. This is a co-op game, treat it as such. Yes, big zeds have a lot of health. That's why you have to worry about them killing you. This is a challenging game, treat it as such.