Unfortunately, if you have objects inside an object (and then in your case, another object inside that object), then you have to shrink each object before shrinking it parent.
SSRS won't be "clever" and scale any objects inside a object which is being shrunk. In your case, having a textbox inside a rectangle, inside a tablix means you would have to:
1. Shrink the textboxes
2. Shrink the Rectangles
3. Shrink the Tablix
If it's a complex report, or have a lot of object, it's not going to be quick, sorry.
Excuse my typos and sometimes awful grammar. My fingers work faster than my brain does.