I also noticed this discrepancy when I purchased Reese's cups from a dollar store and when I purchased them from a local department store (I think it was Kmart).

The dollar store had a smaller 'snack size' than the department store. I was very confused about why Reese's would advertise a single snack size but manufacture 2 different sizes...and then I realized that I had been fooled into thinking that the dollar store size was as large as the department store size. I have also seen some brands use "fun" size and "snack size" to distinguish between products.

Moral of the story: There is a reason why dollar store prices are less, and it's not just because they want to give consumers a discount.