adobe illustrator - How can I change the thickness of a doughnut shape without changing the radius?

I have objects groped together to make a doughnut shape but I want to make the shape thinner. I am using it as a border but my border looks too thick and makes the centre image look small. Is there a way to decrease the thickness of the border objects without affecting the radius?

Here is the group of objects I want to modify.

I essentially want to do this with it:

Thanks in advance for any help.

Answer

With apologies and thanks to @user3164 - whose incomplete answer I parsed in my comment to it and then brazenly stole and then expanded on in this answer...

Take your assemblage of objects, and the fill pattern you've made, turn them into a nice straight (but disturbed slightly) line, and make a New Pattern Brush - you needn't even bother with corner pieces as your specific need is for an ellipse - and hey presto - works like a charm!

I used roughen at 1% on a straight rectangle to get the initial fills, then expanded everything (hence the copy of the fill set) to get a nice clean end edge, then made that into the brush.

Looks like this when you then scale the stroke - exactly as @user3164 suggested!

Oh - and if you separate the body from the salt (or other details) as two brushes - you get additional flexibility...

And you can apply the TWO separate stroke to ONE single geometric entity using the Appearance Palette - like so:

Which lets you then save this as a graphic style... and then apply as you'd wish to any simple stroked item you'd like - like this!

Aaaaand if you're really wanting o finish this out nicely - you then expand your objects and add appropriate end conditions by moving points and adding some judicious curves - sells the results as being more final:

Hope this helps!