I bought a pepper plant in 2021 which produced green peppers about 0.8mm x 2.5cm. I took it indoors for the winter and transplanted back outside the next summer 2022. The peppers were the same size. I repeated the process in the winter. This year, 2023 after transplanting it outside the size and shape of the peppers were completely different. Peppers were on average 5.5cm x 4.5cm (about 15 times the normal size). Can you explain what has happened?

I can't, but I find the occurrence fascinating and hope someone solves the mystery.

Is it a grafted plant?

No, it's not a grafted plant.

Did you plant it in the same spot and care for it (water, fertilizer, etc.) in the same way? How was the weather different in each of those years?

And you are in the UK, correct?

It was planted in the same garden but in a slightly different area. While it was inside during the winter, nearly all the leaves had fallen off. Someone suggested that it may have cross pollinated with another plant.

Are there other peppers in the area that could be responsible? Also are all this year's crop of peppers alike?

Cross pollination only explains it if you saw a brand new seedling grow up.

I planted another pepper tree about 6 feet away but the peppers on that tree are different.

There were no new seedlings.