It doesn't matter if Marcus is made out of metal but rather the weight of that metal. Since Marcus is covered with flesh and clothes, the impact on Kyle is lessened. Also, because Marcus is not physically large or solidly attached to something, Kyle's weight and momentum are enough to push Marcus. Taking a look at the older Terminator models, I'd say (if they're made of a steel alloy) that Marcus weighs approximately 300 pounds.
my physics is rusty, but it's force= mass X acceleration. To compensate for Kyle's lighter mass against Marcus' mass, Kyle would have to generate more force by running fairly fast before impact. Because Marcus isn't attached or stuck onto the ground, he should be able to be moved without Kyle hurting himself.