Horse flies are bigger than black flies .
This from Google :
Horse flies are strong, fast fliers that feed on the blood of livestock and other animals. ... Like the adults, stable fly larvae are nearly identical to the larvae of house flies. BLACK FLIES (Simuliidae) Adult black flies are small, no more than 1/8-inch long with broad wings and a humpbacked appearance.
I recall camping one fall in the U.P. and the black flies were bad . Any uncovered skin was a magnet and fresh meat for extremely hungry critters . There were places we visited that were real bad , to the point of being attacked as soon as we got out of the truck .