"It was hilarious really. Digging them out I couldn't believe my eyes.
"Some of them looked fairly normal, but the big one looks a bit like a tumour or something you've had cut out of your body," he said.
Cowper said he had no clue as to what was the success to his kūmara-growing technique.
"A friend of my daughter's suggested I check for a pulse, while someone else said I should check the radiation in the soil.
"I don't know whether it is something in the climate this year that has triggered it maybe. It's a puzzle."
Despite their size, Cowper said the sweet potatoes still taste normal.
"I gave one of the big ones to the neighbour, but it comes with no guarantee," he laughed.
"I really don't know what to do with them. I suppose we will chop them up, blanche them and freeze them."
Cowper said he plans to expand his vege patch, after retiring last week, but hopes he can grow some normal-sized kūmara next season.