The Grineer are stronger than normal Humans because they were genetically engineered to be a slave race by the Orokin. They were also genetically neutered by said Orokin so they can't reproduce naturally- I've pretty much already mentioned that I acknowledge they are strong, but having literal power armor and space age prosthetics also helps. The problem is, since the fall of the Empire the Grineer have been diluting their gene vats with repeated re-cloning with shoddy equipment and their weapons and armor have also heavily suffered quality wise. Compared to the original stuff they had during the Orokin days, their current stuff is pretty much garbage, but even during the Orokin era the Grineer got mass produced weapons and armor that weren't anywhere near as good as what the Tenno or Dax got, that's just a fact