I have very large calves and what I've done in the past is removed half of the zipper where it's too large and added a gusset (triangle shaped piece of fabric) there large enough so that the boots fit. Just stitch it to the one half of the zipper and put the other side into the boot where you ripped the zipper from.
Here's an example.
The fabric doesn't match exactly because I just used a black stretch that I had on hand and it's a little worn from time, but it does the trick and there's no squeezing your calves into something that clearly doesn't fit. It doesn't look like you'll need that large of a gusset, so it shouldn't be too noticeable, especially if you match your fabric well. You could also substitute elastic for the fabric if you wanted, it's all up to you and what you think will work best.