Seed selection is one of the most important factors. Some varieties are bred or genetically prone to grow larger fruit. Other factors would include Lot's of Sun, a good soil biology, just enough water, a LOT of work, and some luck.

let me list 5 tips on growing larger and huge veggies::

1)prune your plant regularly so that number of fruits remain low on your plant..Those few fruits will get maximum energy and will grow huge

2)Use open pollinated seeds from giant Veggies(Most Important)

3)Provide your plants with steady source of water...It is known as drip irrigation in gardening terms

4)Keep check that your plant is problem free, Eliminate well all kind of diseases and germs problems from the plant

5)Always make use of natural nutrients and fertilizers

I'm a true believer in Miracle Grow. I use it on all my plants.

check my web site. I will be updating with new pics soon.

Here is a link that might be useful: barbara pots & plants

Compost is easy to make, free, organic, an all around great fertilizer and soil conditioner.

i used to live next door to an old man who used a bag off white powder(about a cup full) mixed in 5 liters of water, but he was a bit secretive what it was as he enter vegetable competitions, does anyone know what it might of been?

norah55; My guess would be MgSO4 7H2O Magnesium Sulphate, a saline laxative for the short term relief of constipation, aka Epsom Salt.
There is no substitute for sun and lots of homemade compost,
lots of secret ingredients in the compost to correct soil structure and deficiencies. Water evenly, turn off soaker system timer before/after heavy rain to prevent BER and cracking. Good Luck!
~Ken~