Tomatillos have way more flesh and less water than green tomatoes. They also have starch. I'm going to say something that sounds crazy... but add some granny smith apples to your cook. This adds the starch and flesh needed to bulk it up, and also the slight tartness a tomatillo has (the tang as posted above). The apple will also sweeten but that's a good thing and you'll balance it out with the garlic and peppers.
Don't believe me try some LDHS Dia Del Perro, a verde sauce with apple sauce as a base.