Possibly dumb question: strength without gaining weight?

I've been following the RR for about a month and half and its really really great. So thank you all for your work on it. My question may be kinda dumb, but I just gave never really seen anyone speak of this.

Is it possible to get stronger without gaining weight? I'm training for boxing, and am currently 136lbs, and 5'9. My coach told me after I asked that I should stay at this weight. So what should I do? Could I follow the Reccomended routine and get stronger- without bulking? Thanks for any answers guys.