Jason Whatley, 39, of Fareham, Hampshire, was jailed for five years in May after admitting setting fire to 19-year-old Stuart Mitchell in Leeds.
His sentence was cut to four years at Leeds Crown Court after a judge heard legal arguments from barristers.
Mr Mitchell was left with burns to 12% of his body after the incident in the Headingley Taps pub last October.
Whatley used a cigarette lighter to ignite his cotton wool and Lycra fancy dress costume.
The student needed a number of skin graft operations for burns to his ankles, arms and hands.
"From an early age children are told not to play with fire - every grown man knows cotton wool will catch fire"
Whatley pleaded guilty earlier this year to arson reckless as to whether life was endangered.
Defence barrister Derek Duffy told the court there was no malicious intent involved but described the incident as an "unbelievably stupid act" which went "horrendously wrong".
Sentencing Whatley last month, Judge Kerry Macgill criticised him for "callously" standing by and not helping Mr Mitchell when he was on fire.
On Monday, Judge Macgill said he stood by his previous views but would reconsider the sentence after receiving further information about the case.
He said he accepted that Whatley did not intend the consequences of his actions, but added that he could have done something to help Mr Mitchell as he ran from the pub engulfed in flames.
"From an early age, children are told not to play with fire, not to play with matches, not to play with lighters," Judge Macgill said.
"Every grown man knows cotton wool will catch fire.
"You can say until the cows come home, Mr Whatley, that you did not intend [the consequences], I accept that without question.
"What you did was apply fire to the costume worn by the complainant."
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