Set the max veg to half of what you really want your max veg to be
You see, what happens is that when the number of veggies reaches the max threshhold, they aren't allowed to reproduce any more. If all the vegs on the screen are trying to reproduce, once the number of vegs is low enough, they are all allowed to reproduce in the same cycle, which effectively means that your real population levels end up double the "max".
As to your veggies, that's a very common mutation. First one that I usually see in any sort of veggy sim. If you don't like cancerous veggies, change the nrg feeding method to either per kilobody or the quadratic one.