I once had a section of giant carrots, almost as thick as my forearm. I attributed them to a late spring deluge that washed the top-dressed
compost and sand into the slightly lower lying area. This area also held rain water a bit longer than areas just a few feet away. More water, washed in nutrients, and looser soil due to the sand, something is just a little better in the micro-climate around that particular plant/group. Brassicas store nutrients in their leaves, stems, or roots and if the conditions are just perfect, they can store A LOT of fuel for their next years growth. It's not uncommon for one plant (or a few neighbors) to grow huge, while the rest of the row is normal. Usually, but not always, if you look at the area, you will be able to find the difference that gave those plants the perfect conditions. Better light, more water, washed in organic matter, less competition, something is usually different. Of course there are the occasional genetic variations that could be at play, but that is not as common as conditions being the culprit.