Horses of ALL sizes from mini to draft get the same amount of vaccine.
I usually give my own shots (well actually I am needle phobic, so I find someone at the barn to "harpoon" them for me) because it IS more economical, even if you just have one or two! However, there CAN be SERIOUS risks with giving your own vaccinations so unless you are experienced and have had your licensed veterinarian show you, I'd probably advise against it.
I was lazy this month and had my vet vaccinate my horses when he was here to do some dental work. I will probably regret it when I get the bill, but I do have peace of mind. Horses are expensive no matter how you slice it.
Andrea