Not all Terminator units are huge. For example, the CSM-111 T-806 Recon Infiltrator is child-sized.

Most of the T-800 series that we see are very large and muscled, but that is probably because they were the first T-800 model, and had not benefited from advancements in technology to further miniaturize the components. Alternately, they might have been assault models, meant to be able to sustain heavy fire and return the same from within the base. Other, later, T-800 models have appeared more svelte.

Not all Terminator units are huge. For example, the CSM-111 T-806 Recon Infiltrator is child-sized.

Most of the T-800 series that we see are very large and muscled, but that is probably because they were the first T-800 models, and had not benefited from advancements in technology to further miniaturize the components. Alternately, they might have been assault models, meant to be able to sustain heavy fire and return the same from within the base. Other, later, T-800 models have appeared more svelte.