What makes acrylic paint smooth?

I'm experimenting with paints. I've found that Golden heavy body paints are thick and useful for texture.

But, liquitex soft body paints feel practically like ink in that I can easily draw lines using a liner brush.

Just adding water to the thick paint doesn't achieve the same thing. Is there some sort of material I can add to thick paint to achieve the same effect so I don't need to stock up on multiple types of every color?