I took courses on both differential geometry and relativity, and I am confused by your statement. Relativity is just one application of differential geometry, and I can assure you that Einstein was correct, gravity is a result of the space-time geometry that we live in.
The strongest human athletes... You should probably start by understanding that muscle tissue for vertebrate animals is pretty nearly all the same, human muscle tissue as good as that of any other an animal... the strongest human athletes are top power lifters who weigh around 350 or thereabouts and a maximal total-body lift (squat or deadlift) for one of those guys is going to fall around 900 - 1000 lbs. No herbivore the same size could lift that much for obvious reasons. The weightlifter's body is mainly muscle while the herbivore's body is mostly gut for digesting low value food. The first herbivore which could do anything with 1000 lbs other than be squashed by it would be an elephant.
But you lose power/weight RATIO as you get bigger no matter what you do, weight being proportional to volume (a cubed figure) and strength proportion to cross section of trunk and limbs, which are squared figures. Double your dimensions, and you cut your power/weight ratio in half.
Mathematically, the point at which top lifters become dysfunctional because of that square/cube problem is around 20,000 lbs and the biggest elephants are around 14,000 - 15,000. That's the present size limit for Earth. That means that there has been a very large increase in gravity on our planet fairly recently, and it means that everything Einstein ever said about gravity is wrong. Gravity is not any sort of a geometry thing, it's an electrostatic dipole effect of some sort.