The actor told the Los Angeles Times that the need to delay Clark's transformation into Superman sometimes stretched credibility.
"Each time we got picked up [for a new season] we had to push that finish line further away, and I think we had some low moments when we got too far-fetched," he admitted
Welling also claimed that the series has changed dramatically throughout its ten-year run.
"If you look at the series, the first five years were one show and the next five were a different show," he said. "We could have called [the last five seasons] Metropolis."
He continued: "There were a few times when heroes come in where we allowed ourselves to get lighter. But that's breaking things up. I don't think anyone goes out and tries to make mistakes."
Smallville producers Kelly Souders and Brian Peterson recently confirmed that the series finale will mark the beginning of Clark's time as Superman.
The series concludes this Friday at 8/7c on The CW.
Watch a teaser trailer for the Smallville series finale below: