Why is the iv clindamycin dose more than the oral dose?

Dose based treatment: Iv clindamysin is usually given for severe, often life threatening infections, whereas oral clinda is given for more commonplace infections. Worse infections are typically treated with higher doses of antibiotics.

Doctor's discretion: An antibiotic medication can be dosed a certain way depending on what the doctor is trying to accomplish. For severe infections, a higher dose is often used. And, incidentally, IV forms are often considered 'stronger' than oral forms since the medicine goes right into the blood stream, rather than having to be digested first. It's always good to ask questions- and I'd ask that doc about the dose.