a few months ago my t/b went through a fence cutting his rear leg open.

it has healed, but i have noticed that his leg is larger(swollen) than his other rear leg is this just scar tissue ?

yeah it can be just scare tissue, but if it is really swollen ,I would get a vet to check it cause horse's can need a tetanus shot. If the  fence  was rusty or there is a splinter etc.  the area could be infected.

Scar tissue would create a hard or tough lump around the wound site and might take months to heal completely or never really go away. If its' a soft swelling, it could be infected but appear healed from the outside. Is there heat or tenderness? A poultice that could draw out infection wouldn't hurt and if it's the soft swelling, I"d get the vet out. An infection could go septic (Into the bloodstream) and create problems elsewhere and may need anitbiotics. But don't just start giving penicillin without a vets' advice to do so. Unecessary use can lower the effectivness of the drug and if not needed, just expose your horse to the side affects with no benefit.