But there's a catch. The cans are smaller - 12 per cent smaller, in fact.

Unless you're a born mathematician, different product sizing makes it difficult to compare pricing between the supermarket giants.

So who wins the diet Coke battle? 

Aldi diet Coke is sold in packs containing 330ml cans; Coles sells the standard 375ml. Photo: Edwina Pickles

Aldi's case of 18 smaller cans costs $11.29. At Coles, it is $12.30 for a  full-size 10 pack. Aldi's cans come out at 19 cents per 100ml, compared to 32.8 cents per 100ml at Coles. But Coles and Woolworths sell in packs of 10, 24 and 30  cans of 375ml each  - and you never have to wait long for it to be discounted. This past week Coles was selling 24 packs for $14.10 and Woolworths 30 packs for $20. Final score: Aldi 19c per 100ml, Woolworths 17.8c and Coles 15.7c.

Fairfax Media's survey of six popular products sold at Coles, Aldi and Woolworths found differences between size and price for each one - making it difficult for consumers to easily compare and figure out how to get the best deal. And, as the diet Coke example shows, Aldi, per 100ml or 100g, isn't always the cheapest.

A Coca-Cola Amatil spokesperson said it  works "with retailers to supply products that best meet the needs of their customers," and are able to supply cans and packs that can be modified to a variety of sizes. 

Coles' Milo comes in 750g; Aldi's 385g. Photo: Edwina Pickles

When it comes to Nestlé's Milo, Aldi sold it at one size - 385g - at $1.35 per 100g.  Coles and Woolworths both offered three different sizes - 200g, 750g, 1.25kg. The 750g can is sold at $1.30 per 100g at Coles, making it a better deal.

When it came to Nestlé's Uncle Toby's oats, Aldi was cheaper on the smaller size product.

Aldi sold one size - 660g - for 54 cents per 100 grams, while Coles and Woolworths offered two sizes - 500g and 1kg. While Coles' smaller pack was 75 cents per 100g, the 1kg pack almost matched Aldi's price, per 100g.

Coles oats are sold at 500g and 1kg, Aldi's at 660g. Photo: Edwina Pickles

"Coles offers customers a range of products, some of which are in different sizes," said a Coles spokesperson.

Consumer advocate Christopher Zinn said the difference in sizing was to accommodate different types of people who shopped at the supermarket giants.

"The market is segmented, everyone has got a different optimum of what suits them best," he said.

Aldi has captured 12.1 per cent of the Australian market, trailing leader Woolworths (37.3pc) and Coles (32.5pc), according to Roy Morgan as at December 2015.

Aldi usually offers a single size as they are about the "simplicity of offer", having to find a size that "suits most of the people, most of the time", said Mr Zinn.

An Aldi Australia spokesperson said Aldi tailors its products so it can "meet the needs of tomorrow's consumer".

Coles coffee capsules, sold in 52g packs, and Aldi's, in a 128g pack. Photo: Janie Barrett

Mr Zinn said unit pricing is a "consumer's friend" as it indicates the price of the product per 100g or litre. The inclusion of the unit pricing in stores was introduced in Australia in 2009, but is often not prominent.

Multipacks can also make it difficult to compare across stores. A packet of coffee capsules sold at Aldi and Woolworths contain 16 capsules of 8g each, whereas packets at Coles contained 10 capsules at 5.2g each.

The price per capsule for each box is roughly the same, ranging between 37 and 39 cents, but when unit pricing is used to compare the three boxes, Aldi was a bargain with a price of $4.68 per 100g, compared to Woolworths' $4.91 per 100g, and Coles' $7.12 per 100g.

Aldi cornflakes are sold at 750g; Coles at 500g. Photo: Edwina Pickles

Home brand cornflakes are sold in different sizes. Aldi's box is 750g, Woolworths' is 550g and Coles' 500g. Coles and Woolworths (both 36c/100g) were more expensive than Aldi (33c/100g).

Ian Jarratt, who spearheaded the introduction of unit pricing in Australia, said while unit pricing can be helpful, it's not the most important factor in buying groceries.

"It's all a matter of relative value," said Mr Jarratt.

While families may find that a particular product may be more expensive, it might still be better value because it accommodates for their weekly needs, said Mr Jarratt.

Aldi cake mix, 380g, and Coles cake mix, 340g. Photo: Edwina Pickles

The classic vanilla cake mix sold at Coles and Woolworths come in packs of 340g, while at Aldi the mix is sold in a pack of 380g. Aldi's cake mix was valued at 21.3 cents per 100g, whereas Coles and Woolworths were 22.1 cents per 100g.

So not only do you get a cheaper deal at Aldi, but a bigger cake too.

(Prices were compared at Aldi and Coles at both Castle Hill and Broadway shopping centres and with Woolworths online, which charges the same price as in store, over the past 10 days.)