They are big, tasty questions.

What is the largest fruit there has ever been, and just how big is it possible for fruit to get?

The answer to the first is reasonably straightforward.

This recording breaking feat triggered a discussion about how it is possible to grow such large fruit

The answer to the second, however, is much juicier, attracting the attention of some of the world’s leading plant biologists.

They have just published new research into what influences the extreme size fruit can grow to.

This new research not only reveals something about what goes on inside these giant fruits, it also confirms how much we still have to understand about how plants produce their fleshy, often sweet bounty.

Record breakers

So far, the largest known fruit was a pumpkin, grown by a human, rather than naturally in the wild. Produced in 2014, it weighed more than a tonne, topping the scales at a mouth-watering 1056kg.

The Atlantic Giant variety used today is likely a descendent of the award winning Mammoth pumpkin that held the world record from 1904 to 1976

This freakish fruit is not quite as outlandish as it may first seem.

Indeed records for the largest fruit varieties are broken so often that scientists at Harvard University in Massachusetts, US, decided to research them further to see what could be learned.

“A colleague of ours, Kaare Jensen, brought to our attention in 2012 that nearby in Topsfield, Massachusetts, a new world record was set with a 2009 lb (913kg) pumpkin,” Dr Jessica Savage, at Harvard’s Arnold Arboretum, told BBC Earth.

“This recording breaking feat triggered a discussion about how it is possible to grow such large fruit.”

Descendants of giants

Most giant pumpkins descend from a few known varieties.

“Competitively grown pumpkins were originally bred from Hubbard Squash and their lineage can be traced back through a series of varieties, each progressively increasing in size,” explained Dr Savage.

Some people eat them but they are more often used for decorations or novelty items including boats

“In fact, the Atlantic Giant variety used today is likely a descendent of the award winning Mammoth pumpkin that held the world record from 1904 to 1976.

“However, in the intervening years, seeds from this pumpkin were crossed with many other Mammoth varieties and the exact parentage of older plants is often unknown.”

Novelty items

These giants of the fruit world have limited uses.

Being around 98% water, they contain relatively little sugar and starch, and may lack in taste as a result.

“Some people eat them but they are more often used for decorations or novelty items including boats that are used for racing,” said Dr Savage.

For giant pumpkins, the solution was simple, build more single-laned roads

Because giant varieties are pruned to grow a single fruit per plant, and are heavily fed and watered, it is uneconomical to grow them for agriculture.

“Producing large fruits, especially giant pumpkins and squash, does not always lead to a greater yield per unit of land,” said Dr Savage.

“But they do serve as a great tool for studying fruit growth.”

Pumpkin highways

Dr Savage and colleagues did exactly that by comparing the anatomy and physiology of giant pumpkin varieties to an ancestral variety, with the goal of determining why giant pumpkin plants can produce enormous fruit.

They were particular interested in the plant’s vascular system, the channels within that transport water and sugar.

“We focused on the phloem because it is the part of the vascular system that delivers sugars, which provide the carbon used during fruit growth.”

The scientists discovered that larger fruits didn’t change the structure of their phloem, or the rate at which nutrients passed through them.

It is difficult to say whether it is possible to predict the upper limit of fruit size

Instead they grew more.

“You can think about how giant and non-giant pumpkin varieties differ in their phloem transport by thinking about it in terms of traffic on a road,” Dr Savage explained.

“If more cars travel between two cities, there either needs to be more roads or higher capacity roads with more lanes. For giant pumpkins, the solution was simple, build more single-laned roads, which in the phloem means building more conduits to transfer fluid.

“The actual structure of the phloem cells did not change but the total amount of phloem increased.”

Outer limits

The fact that giant pumpkins create more phloem to transport vast amounts of carbon to their huge fruits sheds light on how plants move carbon around their bodies, and how much of it they allocate to different parts, such as leaves or roots.

What remains unclear is whether there is a limit to how many phloem a plant can produce, report the scientists in the journal Plant, Cell & Environment.

We also do not know, yet, just how big fruits can get.

“It is also difficult to say whether it is possible to predict the upper limit of fruit size, because we do not know what determines when the fruit stops expanding,” said Dr Savage.

While the phloem limits the fruit’s rate of growth, some other factor may finally kick in to stop it growing at a certain size, an unseen barrier that extreme fruit breeders will be keen to test.

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