Hi.
I think you mean cc, not mg. cc is cubic centimeters, and measures the volume. In prostate mass, one cc is pretty much equivalent to one gram in weight (or mass). So we're talking cubic centimeters or grams, because even a newborn has a vastly larger prostate than 70mg.
Anyway, as you note, your prostate was very large even at 70cc.
I don't think you have to worry especially about
cancer, just because of the prostate size. This study, for example, found that "the majority of men with large prostates (>70 cc) have PSA elevation due to benign prostate hyperplasia (BPH) and not to cancer."
www.ncbi.nlm.nih.gov/pubmed/12667711 and there is no evidence that benign enlargement causes prostate cancer or somehow turns into it.
It doesn't mean you don't have or won't get prostate cancer, but the size of your prostate by itself is not a risk for it. Hopefully your biopsy will be negative for cancer, and you'll get some effective treatment for the BPH.
Good luck.