You won't like it, and it's undoubtedly not fair (or perhaps even rational), but she doesn't look like a leader (just as Corbyn didn't). Look at the evidence, and most political leaders are tall. I think something like 90% of the winners in US elections over the last 100 years have been taller than their opponent (and I say this as a short guy - it sucks, but it is what it is).
https://www.economist.com/business/2014/09/27/the-look-of-a-leader explains the various biases that creep in when society selects leaders more eloquently than I could on a Sunday morning. It's why Corbyn's Lenin hat and scruffy jacket at the cenotaph were such silly and avoidable own goals. You could even say his beard worked against him (can you think of a political leader with one outside of latin America?).
These are undoubtedly shallow ways of judging someone, but we must surely realise by now that most peoples choice is based upon fairly shallow reasoning as they're not as into politics (by a long way) as you and I.
Johnson is quite short though. And some of the best leaders of our time, Atlee and Thatcher were also short. I don't remember Churchill being particularly tall either. Eden was though....