If you want to go "large", beware - upsizing your fries in fast-food restaurants does not guarantee you much for your extra money.

University student and McDonald's regular Allen Hsu wanted to find out if he was getting value for money when he up-sized his medium McDonald's combo to the large size after he became concerned at the similarity between the two sizes.

The 18-year-old counted the number of french fries in three packs of medium McDonald's fries and three packs of large fries and found his initial suspicions proved right - there were only 14 more french fries in a large size of McDonald's fries compared with the medium size.

Mr Hsu also found one pack of large fries had only one more chip than one of the medium servings.

"It's 70 cents extra to upgrade the fries so my friends and I decided to see if it was worth it. I don't think it's worth it any more for an extra three or four fries," said Mr Hsu.

Independent tests conducted by the Herald yielded similar results to Mr Hsu's experiment, finding there was only a 12 french fry difference between the medium and large sizes at McDonald's and, in terms of weight, only a 13g difference.

A medium french fries is $2.60 and large is $3 - to up-size a combo costs 70c.

McDonald's communication manager Christine Dennis said the fries' box size determined the amount of fries served as restaurant staff were trained to fill the packs. Restaurant kitchens also had diagrams beside the fries station showing the correct filling amount.

However, there could be some variability because of different fries' lengths and sizes, human variability in shaking the servings into the box, and because staff may have tried not to crush the fries by squashing too many in one box.

Every year, around one million New Zealanders eat at a McDonald's restaurant, making their way through 15.6 million kg of potatoes that are used in the production of french fries.

The Herald also found another popular fast-food chain failing to give customers their money's worth in french fries.

On average, there were the same number of fries in Burger King's small-sized french fry containers as in the medium-sized containers despite the 40c difference.

Burger King communications manager Rachel Allison said their medium-size french fries container was designed to hold 35 per cent more than the small size but this could differ depending on the server in each restaurant.

However, the Herald found misleading sizing is not a problem for Restaurant Brands-owned KFC.

Their large size of french fries have almost twice the number of fries and twice the weight compared with their small size.