Make the undead unable to give new life. Basically raised people can have sex, but they can't have children.
Now you might think that this still makes for a bigger population and you are correct. But it is not that huge of a difference. Math is on your side.
To make calculations easier, we'll assume that child mortality is not an issue, people form lifelong couples and the ratio of men:women is exactly 1:1. We'll compare a world where necromancy doesn't exist, with the one where it does with the above conditions.
let x0 be the number of people immediately after the apocalypse
let c > 2 be the average number of children per pair
let n be the number of generations we will look into the future
Then the number of people without necromancy is:
And the number of people with necromancy is:
Which is a simple geometric progression sum, hence:
Now lets examine how fast z grows compared to y:
This means that with 3 children per family you will get up to 3 times more people with necromancy than without. With 4 children - up to 2. With 5 children - up to 1.67. And so on. 838
Here is some code if you want to play around with different values.
Now, 3 times more people might sound like a lot, but it's really not.
tl;dr in case you don't care to follow the math:
If 3000 people start reproducing and don't use necromancy, no matter how many generations you look ahead, there will always be more people than if 1000 people start reproducing and raise every single person that dies, given there are 3 children per family on average. If there are more than 3 - the numbers are even more in favour of the no-necromancy folks. Evident by this graph.
However, this required us to ignore infant mortality. One way to deal with it is to make raising babies/small children illegal and/or frowned upon. This doesn't look that far fetched given:
1. The undead body is preserved by magic.
My interpretation is that raised people don't get older and don't grow. Hence no one will want a couple to look after a crying infant for the rest of their lives and they will be nudged to try to conceive again instead.
But even if you raise infants, the conclusion will still hold. It just won't be 3000 vs 1000, but 10 000 vs 1000 for example. 838