They are the unspoken tricks of the trade - the subtle techniques manufacturers employ every day to get shoppers to pay more, for less.

And more often than not, they pass unnoticed.

They can range from widening the opening in your favourite bottle of tomato sauce or tube of toothpaste, so you use a little more with every squirt, to tweaking the size and shape of the product while the price remains the same.

Recent examples show how these tactics have been employed in Australia.

Kellogg's, this year reduced the size of 38 brands by two servings a box - including Sultana Bran, Nutri-Grain and Rice Bubbles.

In 2009, Cadbury reduced the size of a 250-gram block of Dairy Milk chocolate by 50 grams - a 20 per cent reduction.

Likewise, a re-jazzing of Pantene shampoo packaging had the size of bottles shrink by 50 millilitres.

Sometimes the firm gets caught out, as Foster's found when it tried to reduce the size of Cascade beer stubbies from 375 millilitres to 330ml "European-style" bottles - while keeping the good old Australian price.

A consumer backlash soon forced the brewer to fatten the bottles back up.

So whether the technique passes unnoticed or not depends on the appearance. An old advertising maxim is that nothing matters more than the package.

"The package is the last five seconds of any good campaign," said University of Sydney Business School marketing professor Charles Areni.

And so packaging becomes one of the strongest tools in a marketer's arsenal to trick us into consuming more, more often.

The 'large-opening' approach

Deceptive or not, we continue to buy - even after realising we've been duded.

"A fairly straightforward and legitimate way to increase sales is with a large opening," said Ken Miller, emeritus professor of marketing at the University of Technology, Sydney.

"But customers get savvy about this pretty quickly," he said.

Marketers have used this technique on tomato sauce, toothpaste and soft drink cans in Australia, thinking that people will finish a product more quickly and therefore buy more.

This has been done in both subtle and obvious ways - the latter exemplified by the soft drink Solo, which marketed the enlarged opening on its can to allow the drinker to "slam it down" even faster.

The subtle approach is more common, but said Dr Miller, this is a quick fix.

A person who accidentally squeezes too much sauce out of a bottle because the opening is 10 per cent larger will not make the same mistake again.

"You might get caught the first time, but you soon realise you don't need that much," he said.

Playing with your mind

But quantity does not always play a role in consumers' decision making. Sometimes, shoppers will take their cues on what is an appropriate amount from the way food is packaged.

Dr Areni said Tim Tams were the best example of a company - in this case Arnott's biscuits - using packaging to encourage a customer to eat more.

"Unlike other cookies, that are packaged in partitions, these are packaged in a row - so you go right the way through," he said.

"Other biscuits have a series of rows, and it implies that is the stopping point."

The good news for marketers is that, as far as memory goes, we as consumers are like goldfish.

"To predict what a person will buy, the best indication is the list from their previous shopping trip," Dr Areni said.

Product downsizing

Making package sizes slightly smaller - while keeping prices the same - is one of the most popular ways to encourage you to throw more items in the trolley.

As shown above, Cadbury, Pantene and Kellogg's are all companies that have subtly downsized.

CHOICE spokeswoman Ingrid Just said this tactic was used because consumers generally were more observant of price than they were of size.

"This is why we would say always check the unit price," Ms Just said.

- Sydney Morning Herald