Yeah, the dosage covers a wider weight range. My old dog was 50 pounds and I gave her the Valuheart Large dog which as you said goes from 44 pounds to 80 pounds. She never had any problem with the dose being strong enough for a larger dog. My current young dog is 38 pounds and gets the Medium Valuheart which goes up to 44 pounds so that dose is a bit closer.
For the once a month heartwormer use, I don't think the higher dose harms the dog. If I remember right, it is dosed much higher and more often for treating demodex mange.