Based on firsthand experience, the Australian cattledog (please don't confuse with the Australian Shep) is a robust livestock guard breed in a small resource-efficient package. They have stamina for days, are eminently trainable if you start at an appropriate age, consume less food than larger breeds, and have the proper drive to be trusted with rounding and protection tasks even in absence of a handler.
Mine have gone above and beyond to repel predatory threats as large as black bear, often when I was away from home and my herd of goats would have otherwise been vulnerable. I've had several and they've all been healthy, hardy, and easily put on task. Yes; a larger breed would have more combative capacity. But they also require pounds more of feed a week compared to the cattledog, which is a very important concern when I have to potentially supply nutrition for myself and every living animal in my care.