Is smoking one joint really the same as smoking a whole pack of cigarettes?

No: That is an often told and totally fabricated "fact" whose origins are unknown, but there is no evidence for it. The "tars" or organic compounds contained in marijuana plants is different than those in tobacco, and does not appear to be as harmful. A recent study (2012) showed that "social" marijuana smokers did not have any evidence of pulmonary damage, compared with smoking same no. Cigarettes.