if you want certain muscles to shrink just stop using them. You could also do low intenisty endurance excersize eg long distance cycling, and then not eat enough protein, only problem is i do not think you can use a method like this to shrink muscles with out shrinking all of them, so just stop working on them and they will shrink a bit, notice how peoples muscles get so small if they break a bone and get a cast so they cant move that part of their body.
If you mean huge as in lots of fat, well there is not way to localise fat burning just excerisze more and dont eat high fat foods. Some people may say do low intensity excersize it burns more fat, high intensity just burns carbs, well actuly because total eenrgy output increases so much even thou a high % of whats burnt it carbs more fat is burnt then at low intensity, so high intensity will always burn more fat.