A larger majority would help May in several ways.
It would make it easier to pass legislation that would deliver a clean or "hard" Brexit by regaining control of borders and the budget, rather than safeguarding trade.
Tourists cross Westminster Bridge beside the Houses of Parliament in London, U.K., on Thursday, Dec. 29, 2016. Demand for luxury brands in the U.K. is flourishing, boosted by increased tourism and spending linked to the weaker pound.
"There was not a majority in the Commons or Lords for a hard Brexit," said Mujtaba Rahman of the Eurasia Group. "If, however, as we expect May secures a bigger majority, she will have a people's mandate for a hard Brexit."
At the same time, a larger majority would give her room to make concessions forced upon her by her EU counterparts and which the most vocal Brexit campaigners in her own party might not welcome.
"All else equal, that should lower the risk of a very disruptive Brexit as the government should be able to plot a less confrontational exit," said Mike Amey of Pacific Investment Management.