I don't think larger dogs can do the same as a 60 pounder. Maybe on a limited individual scale, but not as a general, global breed scale. There's a reason that larger dogs break down faster and are not as sought after as smaller ones. You don't hear stories of people taking their GSD's on a 20 mile run behind a horse to compete at a schutzhund trial then run back the 20 miles again after doing OB, protection, and tracking at the age of 10 years old, but they used to.
The breed didn't used to have to be "retired" from work by age 7 or 8 like they are today. There's a reason for the standard and I don't know why people think they know better. Trust me, I get hit by the 50 and 60 pounders just like I do the big ones. I know what they feel like. If I wanted a big heavy lug to just hang on and stop someone, i'd get a rottie. If I wanted a strong agile, rocket, climb up ladders, scale walls, work all day work past age 10 and live a long active life i'll take a "small" gsd, mal or dutchie. Bigger isn't better
measure your dog like this
http://www.sonic.net/~cdlcruz/GPCC/library/measure.htm
the height for a GSD bitch is 21.6- 23.6 inches tall, or if you like cm 55-60 cm