I had one and have been around loads that were around that weight/Size and your right most aren't able to get there, I'd say 75% of how my boy can get like that is in his genes. His sire is 95lbs and considered a freak for his line as they're usually 75-85 dogs and 65-75 lb bitches and he has the same physique and can go all day like my boy.
How's your bred ? Mine has no Johnson blood in him so no dodgy out crosses to English Bulldog, St Bernard, Mastiff, Boxer etc which will affect stamina/working ability etc and hes been bred with working ability, drive and temp above looks/size he's ju