The phrase "a very big factor" is correct and usable in written English.
It can be used when discussing the significance or importance of a particular element in a situation or decision-making process.
Example: "In determining the success of the project, funding was a very big factor that influenced our approach."
Alternatives: "a significant factor" or "a major factor".
Charles Yulish, a company spokesman, said he believed that Russia would be a stable supplier in coming years because the company's payments, about $500 million a year, were "a very big factor in Russian revenues".
"That was a very big factor for me," she said.
That was a very big factor for me at a time when employment isn't guaranteed".
"But the poor performance of the EU economy is a very big factor".
In athletics, injury rate is a very big factor, and we are in a good place at the moment".
"The write-in votes played a very big factor, but a win is a win," Trump wrote.
Only a few hours before he visited Geno's Steaks in South Philadelphia, Trump told a crowd – and rolling cameras – in Pittsburgh that "drugs are a very, very big factor in what you're watching on television at night", referring to riots against police abuses in Charlotte, North Carolina, the night before.
"And if you're not aware, drugs are a very, very big factor in what you're watching on television at night," Mr. Trump said.
The Republican nominee suggested earlier in the day in Pittsburgh that protesters may have been acting under the influence of narcotics, telling the Shale Insight Convention: "If you're not aware, drugs are a very, very big factor in what you're watching on television".
Paltry though they may have been in the past, Isner said the absence of ranking points was "a very, very big factor" in his decision not to play in Rio, as was his not wanting to miss the ATP tournament in Atlanta, where he has won three titles in a row.
One very big factor was a TV ad he aired that featured his son, Dante, talking about his father's stand on the issues.
