Consider a sphere with physical and chemical properties favorable to your solution (for example responsive to electric force or high heat capacity or anything). How would you compress it such that it remains a perfect sphere and simply faces a change in radius?
You can't extract material from inside, so obviously you'll need to increase its density. Hence we aren't considering the extent to which you can achieve the decrease in radius: you can assume something small like 1%.
I've been thinking and I've decided that the only way to achieve this is by applying the same force on all sides, but after this, I got stuck. Maybe placing it in a mold of the same shape and then applying the thermal expansion of the mold to shrink the ball? Are there any other interesting ways?
I'm considering this just as a thought experiment, with no concern for feasibility, so feel free to use unorthodox techniques.