Size (well mass) and the fact that they're airborne. "I hit a squirrel" "I hit an eagle" I'd guess the size is about that of a chicken. "I hit a chicken" or "A chicken hit my windshield" seems about 50/50. "A pigeon hit my windshield." "I hit a cockatoo." So maybe just a little smaller than a chicken. I'd say if it was big enough to feel the impact you hit it.
However i dont think its just that. It’s whether I could have avoided it by steering.
If it hits my windscreen, presumably there was nothing I could have done about it. It hit my windscreen.
If I drove over it or into it with the front of my car, that’s usually regarded as within my purview and requires a special explanation if I am to be excused of responsibility.
Indeed, I would add as further evidence for this theory that if the bugs accumulated on my front grill or headlights instead of my windscreen, I’d be inclined to say that I drove into them. Even though I can’t really have avoided it, it’s the part of my car where we usually assume it’s my job to keep it from contacting stuff.