What you can do to affect how many of your pops are raised is change your economic policy for your armies, these can increase or decrease the portion of your integrated population that you call to arms.
Laws also affect the percentage of people levies, with republican laws especially allowing you to levy a larger part of your population, should you so wish. Each tradition tree also contains a 5% levy size buff should you choose to go for it.
Lastly Rome rather famously was able to raise quite big armies from its citizens, this is reflected in an additional levy size % boost for the Roman heritage in particular, which is part of the big levy you can see the in the screenshot. It is a bit of a double edged sword as you say