Yoga doesn't lengthen the muscle at rest it simply makes it more flexible, the length is determined by the attachment points on the bones, and bones don't generally grow in adulthood. Before you consider muscle reduction surgery if such a thing is possible, I would suggest you spend a lot of time researching the structure-function relationship of each head of the triceps and biceps muscles. If you mean the part of your arms that makes your shoulders seem broader, you might be thinking of the deltoid so research that also. The biceps and triceps flex and extend the elbow joint AND assist with pushing and pulling moves AND movement of the arms generally aids balance. You probably won't be able to open a door, lift a mug of coffee or glass of water, shower, push a shopping trolley at least in the early stages of recovery. You really want someone to take you to the bathroom, pull your trousers and underwear down, wipe your backside? Can you stand up from a low sofa or get out of bed without pushing with or swinging your arms for momentum? A LOT of people cannot.
If you want to reduce the muscle mass naturally try combining a reduced calorie/ low protein diet (within medically safe boundaries remembering the heart is a muscle), plus exercise for the lower body to maintain as much muscle there as possible. Use the arms and shoulders as little as possible, which will likely mean only driving in emergencies and taking time off work, this will be far more effective at persuading your body to cannibalise itself than low weight high reps. Not saying it's practical or fun but it's likely safer than surgery.