They had a much smaller majority because of all the votes/seats gained under Corbyn in 2017. May went to the polls looking to increse their majority butand until Corbyn's manifesto was released the polls were showing that it would happen. The manifesto and Corbyn's performance in the run up to that election meant the only way that May could even command a majority was by paying off the DUP. The 2017 election was weirdly not one where Brexit got a lot of air time. At the time both parties had the same position on Brexit so Labour's (surprisingly good) performance showed that there was genuine desire for that Labour agenda and if the majority of Blairites in the PLP had been onside then it is an election that Labour could have won or at least prevented the Tories from being able to command a majority even with assistance. They were happy to let the country go to the dogs just so Corbyn didn't get a chance to govern and anyone supporting them for that should be utterly ashamed of themselves.He did defeat lots of votes, however there was a much smaller majority for the tories and it was during brexit when the tories were voting against their leadership also.
The 2019 election was completely different. Corbyn was weak, undeniably, because he should have stood up to the remainers in the party and made them accept that Brexit was going to happen instead of ending up with his non-position. The manifesto was irrelevant in that election. It was a single-issue. He knew it was too late for him by that point. He knew Brexit was going to happen as it is what people wanted but he had agreed to do what the membership wanted and the remainers insisted (unlike when the membership wanted Corbyn).