I think in normal circumstances full gas cans could be heavy. Not too heavy to lift, but still quite a bit to lift up in a way necessary to fill a car's gas tank.
However, these were not normal circumstances. This was a woman scorned, hellbent on murder. Nothing was going to stop her. Nothing.
Also, hasn't it been said by people who knew her that Arias was exceptionally strong physically? Seems to me I heard at least one of her acquaintances remark on that.
BBM - Yes, RB for one. There's also a little something that usually comes with gas cans when you buy them.
http://www.cnbc.com/id/101256842