There is no percentage for the brawl boxes ( the regular ones you get from tokens). The big box (which you get from star tokens) has the same odds as the Big box in the store. Then there are mega boxes you have to buy or get from trophie road
Rewards are as follows:
Brawl boxes 1x rewards
Big boxes 3x rewards
Mega boxes 10x rewards
So you are 3x more likey to get a legendary brawler from a single Big box
And 10x more likey to get a legendary from a Single Mega box
But its the same ODDS as 3 or 10 regular brawl boxes you just don't have to wait or work to get them.