I've found this method quite effective. 'Dealing with larger opponents 101'
(Works best on opponents greater than 6'2'' and 250lb)
Wait until your attacker closes to between 5' to 3'. Then as he opens his mouth to ask for your wallet, wedge a small set of stepladders into his open mouth, rendering him helpless. Thus allowing you to make your escape.
If, however you are on the street and a set of stepladders are hard to come by, then a bedside cabinet or computer table should be just as effective.