The six-cap Wallabies centre spoke publicly for the first time on Tuesday since his move to Benetton on a two-year deal was confirmed last month.
Flook, 24, was due to link with the United Rugby Championship outfit last year on a secondment in partnership with the Reds.
But he was pulled into Wallabies camp for their end-of-season tour and then deployed just once, against Japan.
"Talking to some people, the feedback I got was that I wasn't really sure how I'd match up against the Scottish and South African centres, if I was to go up against them," he said.
"So I thought, 'Why not go and test myself in that (URC) competition and prove to myself and others that I might be a little bit smaller but can put up a hell of a fight?'"
Flook, whose strengths are timing, angles and smooth skills, said being challenged about his lack of size was nothing new.
"I've been told that all my life; it's a driver for me," he said.
He said it was the "toughest decision of his life" to leave Super Rugby Pacific, from which a large majority of the Wallabies squad for next year's home World Cup will be selected.
But he thinks exposure to the Irish, Italian, Welsh, Scottish and South African URC teams could ironically aid his Test chances.
"I mulled on it for weeks and weeks, probably pissed a few people off," he said.
"But I'm only 24 ... whether I get a gold jersey or not, as long as I'm improving and bettering myself as a player and person."
Flook returned from injury in Saturday's disappointing 42-19 loss to the Western Force.
That followed a lacklustre loss to the Hurricanes in Wellington that has dropped the Reds - who could have sat third with a win in Brisbane - to sixth.
Queensland (4-3) host the Crusaders (fifth, 4-3) on Saturday before a bye is followed by Super Round's Christchurch clash with the Blues (second, 5-2) and home games against the Brumbies (fourth, 4-3) and Chiefs (third, 5-2).
There is an extra layer of scrutiny on the Reds' results, with coach Les Kiss set to take over as Wallabies coach following the Super Rugby Pacific season.
But Flook said their mid-season stumble was not a reflection of his coaching.
"There were lots of individual looks in the mirror and then we've looked as a group at what can we do better and what little bits of our game are we missing," he said.
"It's definitely unfair to put Les in the spotlight like that.
"The last two weeks has shown, if we make mistakes there are good sides able to capitalise, but if we hold the ball we can play really well."
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