Gee, it's really a headache for sweaters to get bigger. I usually do it in the following way.

first step

First, prepare a basin of warm water and put the sweater in it to soak.

Second step

Next, you need to use mild detergent and hand washing to clean the sweater.

Third step

Finally, after washing, wrap the sweater with a clean towel and steam it in a steamer for 10 minutes. Then, rinse with clean cold water and dry it.