Sometimes the absorbable stitches can take longer to break down in the wound and will cause a temporary lump in that location. The middle stitch can also get a little drippy which suggests that the wound was infected and involved significant tissue damage (which is the case for all traumatic wounds). Part of the natural healing process includes the body eating damaged tissues and replacing them with dense scar tissue. When the healing phase is prolonged by a factor such as infection or excessive movement, a larger mass of scar tissue tends to develop.
This is very evident in the lower limbs, where skin lacerations are hard to immobilize and proud flesh rapidly develops. Sometimes a little foreign material left in the wound can cause thickness and drainage, but your vet did a great job cleaning and stitching it up so this is very unlikely. Over time the lump on your horse’s leg should diminish, and as long as he is not sore over it or you start to notice intermittent drainage from the scar, I wouldn’t worry too much about it. You may wish to talk to your vet about topical treatments that could speed up the process of healing, such as DMSO gel.