Original thread at Ludeon forum.

As you already know (or don't) RimWorld has a population cap depending on which storyteller you pick. Cassandra has a critical cap of 18, Phoebe also 18 and Randy has a critical cap of 50. This mod makes it so that you are more likely to get new pawns at higher population levels and also makes the default cap double, so you can have more slaves at hand at the same time.

It makes use of the new patch feature. Learn more about it here: here.

Also note that this is my first mod and I may not provide any support for it. I've made it, it's simple, it works (probably), enjoy. 

You are free to do anything regarding this mod.

If you want to make changes go to your Mods folder, DoublePopulation, Patches, patches.xml and edit values between <li> and </li> tags. Note that there are values for all three default storytellers.

    <Operation Class="PatchOperationReplace">
        <xpath>*/StorytellerDef[defName = "Cassandra"]/populationIntentFromPopCurve</xpath>
        <value>
            <populationIntentFromPopCurve>
                <points>
                    <li>0, 8</li>
                    <li>2, 4</li>
                    <li>10, 1</li>
                    <li>18, 0.5</li>
                    <li>26, 0</li>
                    <li>36, -1</li>
                </points>
            </populationIntentFromPopCurve>
        </value>
    </Operation>

What these numbers mean:
If you are at 0 colonists, you are 8 times as likely to get new colonists as you are by default. If you are at 2 - 4 times, 10 - 1 time etc. 0 is the normal cap, which means you will not get any more free colonists and -1 stands for the previous "critical", which means you will have much harder time recruiting prisoners, roughly.

If you don't want to alter the default storytellers values there is also a download for additional storytellers.