I may also equates to being presented with a half kilo box of chocolates and a half kilo bar of chocolate.
i could eat both in one sitting

, to be very honest i once ate a 2kg bar of cadbury's dairly milk (don't think they do them anymore

) whilst watch indiana jones and the temple of doom

and think nothing of eating a family sized choclate trifle (extra specil one's are the best by the way

), whilst a 500g pack of crisps chicken and thyme flavour would pose no problem at all, and before anyone asks i weight 10 stone 7 pounds and have done for the past 20 to 30 years give or take a couple of pounds, and mmm chips cooked in beef fat with lashings of salt, pepper and vinger now im hungry
Anyhow back to the question in hand i agree with shadow it has to be a mind thing, although i did think along the lines of the quantity of starch and carbs in the helping, would that change if it was cooked in a diffrent manner ie: fried or boiled or baked

In life you make mistakes, but that's life. As long as you learn by the mistakes you make, don't be afraid to make them.