I think you're just discovered a "downsizing". This word has many explanations and definitions depending on aspect of life, where used. But in influence and manipulation the meaning is clear and partially explained by Josh. Your brain assumes the bigger package is, the bigger value it has or the bigger contents you're getting.
The general idea of downsizing in influence and manipulation is that you're getting the same amount as in case of competitive product, but you think you're getting more because of bigger package. An alternative version of downsizing assumes that you're getting less than competitive company offers, but you think you're getting the same, because of the same packaging. And — of course — the price is the same as that of the competing company. Thanks to packaging, it does not have to be adjusted to lower amount customer is getting.
The key to the success is that in most cases, the differences between reduced and competitive products are so slight that customers don't realize they've bought less paying the same (not adjusted price). But since the not noticeable difference is multiplied by thousands or even millions of sold products, customers see no difference and the company saves or earns extra millions of dollars.
This works in every or nearly every aspect of shopping. If you'll see two milk shakes, two washing powders, cakes, butters, milks, anything and one of them will be in a bigger package than the other, you'll most likely automatically pick up the bigger one. Your brain assumes that this is better for you. LEGO is no exception here and I'm pretty sure that if they wouldn't receive negative response and push from eco-oriented organisations (what RegDwight explained) they'd continue their politics.
This works brilliantly (for the manipulator, not for you, of course), especially if you're doing quick shopping in a large mall and you reserve seconds or even split seconds for the process of selecting a particular product over another one. You're often fouled by manipulators using downsizing — bigger packaging does not necessary mean higher volume. I saw (too) many examples where the customer selecting a bigger package was in fact getting a smaller volume. But due to the very limited time for decision, they only checked the size or weight at home, or [erhaps never.
Some sources claim that a big pinch in the bottom of each or nearly each wine bottle is in fact another example of downsizing, not a handy catchpoint allowing the bartender to hold the bottle in one hand. If you pinch-up the bottom of the bottle that much, you can produce a bigger (wider) bottle, that contains the same amount of wine a competitive company offers. But since your bottle is wider (thanks to pinch-up) your customer will most likely choose your bottle, not the competing one.
Downsizing in influence is sometimes called "A British Airways olives example" after the first documented use of this mechanism. Which, at the same time, explains why it is so common and so often used to foul us. Many years ago, British Airways managers calculated that removing two olives from drinks served on board all their planes would reduce the overall costs of flights by millions of pounds in a year's time. They went ahead and thus downsizing was born. Customers were getting less than competitive flight companies offered, but the difference was not noticeable, so nobody complained. When this small difference was multiplied by a number of flights British Airways serves on each routers during entire year, company savings were at level of tens of millions pounds.
