You are sort of looking at it wrong. The stuff below Df 4 does not matter. It is only the higher end Df that matters. The enlargement is not about not being able to hit consistently it is not being able to hit them at all. I do not have my books, so I can not actually look at the models in the Rezzer faction that have such high Df, so I will take the numbers at face value. However the numbers that matter are the Df 5+ and more importantly the Df of 6 or 7. Not being able to hit at all is the thing that is problematic. Rezzers have very few if any models that are typically able to be saved. If you have a Df of 6 then a lot of models can not hit you at all if you have high cards in your hands.
The other thing, which I can not do that good of a job defending without my books, is what value of the models with the high Df. Belles have typically low Df, but that is fine, they are there to take hits, they are designed to take hits. I can not think of many Rezzers that should be avoiding hits (mostly Masters) with High Def. Bette is the only one that comes to mind. Neverborn is the opposite of this. They have a lot of fragile models, but they are very hard to hit. Sometimes impossible to hit. Most of the models they need to keep alive, they have the opportunity to actually keep alive. It takes the risk out of things. Even with hard to wound 2 you can still take severe damage, and you are probably going to be taking at least 2 or 3 damage every attack. So on average 5 damage a turn. With very high Def you are avoiding damage and all together. Avoidance is much greater then mitigation. Everything in the 3-5 range is honestly a wash in my opinion as most things will have stats in that range and the randomness of the cards is a bigger factor then the cards in ones hand.
Finally, and this sort of has to do with avoidance, is that really high Df is more then likely to make you be the one that cheats second. This is incredibly advantageous.
I do not have a problem with Rezzers as they are built, but I think the design philosophy does not fully take the difference of mitigation and avoidance into consideration.