Benchpress Question?

Ok i've been working out for the past 2 months now, and i'm using a pretty cheap home gym set, but its working. I'm using a 20 pound Bar, with those cement-filled weights, the 20 pound bar is really short and thin. And i can only benchpress up to 70 pounds using that bar, but i can benchpress 160 on a 45 pound bar. whys that???