Based on these numbers a bigger majority was almost a certainty
Almost whatever Mrs May does in the rest of her career she is going to go down as the PM who called an election when she had a majority and ended up without one. With the benefit of hindsight it looks like a massive mistake.
Yet looking at the polling from the 6 weeks before her dramatic decision who could blame her for believing that this was a almost risk-free move? LAB had just the Copeland by-election to the Tories was in the 20s in all but one of the survey while all but one of them had CON in the 40s. The margin was enormous.
I doubt whether someone as cautious as Mrs. May would have dared take the plunge if the blue lead had just been in single figures.
When the first spread markets came out they were pointing the a CON seat total of 400+ and a majority well into the hundreds. Even on the the evening before the election some were predicting that scale of victory.
What happened in the election was the collapse of UKIP and the LDs not performing anything like at the level the March polls had been suggesting. The big beneficiary was, of course, Labour.