There is no direct way to shrink the collar when the T-shirt is enlarged. That's how I handled it. I wonder if you can do it well after telling you. Specific operation method: 1. Slowly remove the neckline (don't pull it, or it will be deformed), make a mark on the original neckline with the line as the starting point, compare the rib of the removed neckline with the original neckline, and then reduce it by1.5cm.2. Splice the two broken ends of the rib into a circle. 3. Sew the rib of the neckline and the neckline with zigzag on the multifunctional sewing machine. The neckline treated in this way will basically not get bigger, but I always treat it like this when the neckline of knitted clothes gets bigger. I am a clothing major, so this kind of thing is not a problem for me. I wonder if this is a problem for you. I hope my answer can solve some troubles for you.