The Hartgard dose is low because they are trying to avoid dogs with the MDR-1 gene problem dying. Also, if you look at how it is dosed, some of the weigh ranges are double at the top end, so the smaller size dog is getting double the dose of the larger size but it is still fine. Ivomec has a HUGE range of safe dosing. Dogs receiving it as a treatment for other issues are getting insanely high doses compared to a monthly Hartgard tablet.
I give my 70lb adults about .35cc of the 1% injectible. I did the math with a chemistry prof and that's what I feel works and what I'm comfortable with. It is a considerably higher dose than a Hartgard tablet, but not as high as the .1cc per 10lb dosing I see thrown around online. My dogs always test negative for HW and I also used this dose to prevent the spread of roundworms when my new puppy came home with them.
FWIW I'm not a vet so consult with your vet.
Coke (All-American 7/7/06)
Nikon (GSD 9/7/08)
Indy (All-American 5/10/12)
Legend (GSD 10/22/13)
Rainbow Bridge Kenya (GSD)