Bigger is not better. Bigger is dumb for Terminators.
Connor will likely be harder to find in the future (no pun intended), he fits in with any storyline yet because noboby has gone to the future on screen yet with John, and that likeness just makes him a perfect displayable piece.
the T-600 is cool, but doesn't fit the Cameron mythology that the only difference between 600 and 800 was rubber skin.
Kyle said, "the 600 series had rubber skin, we spotted them easy, but these (800s) are new, they look human."
He didn't say, "the 600 series was gigantic like the Incredible Hulk with huge gatling guns strapped to their arms and rubber skin, we spotted them easy, but these (800s) are new, they only look like Austrian Mr. Universes."
Besides, Connor is cheaper.