Gracia will test the real power at 147 which can have a big impact on him. He is leaping across 2 weight classes which could lead him to a devastating KO loss in this fight.
In fact, these "odds" are a coefficient (multiplicative factor) to determine how much you can win. So, if for Spence it's 1/4, you multiply your bet amount (let's say it's $100) on it: $100 * 0.25 = $25 Of course you get your initial $100 back as well if your bet is correct.
The height difference might/may give Spence a harder time in hitting Garcia. Kindda like the Pac vs. Margarito. Just saying....
Yes, deter gamblers from betting on the fight. Real smart move on the bookies part. The "Vegas odds" will always reflect how bets are made. At 4-1, people will be incentivized to bet on Spence, because there is some return on the bet. And the Garcia fans see big value on him. And if Garcia does win, the casinos won't have to pay out a freaking fortune.