Well considering the Sith Empire had a tiny portion of the galaxy compared to Republic before they attacked, they certainly had less resources and manpower. But they had better prepared and better trained military. Things changed after the war. Sith increased their territory but the Republic could draft in more troops and train them. So after that it is hard to say. But at the beginning Sith surely had better military and superior tactics to counterbalance their lack of resources and numbers.
I don't know if I'd agree with the superior tactics.
"You failed! Die!"
"You saw what happened to your superior, and now you've failed as well! Die!"
"You lived and your superior died in that battle. Die!"
I'm actually curious why there aren't more defectors or more Sith shot in the back.