Because a lot of vegans are leftist nerds, who are often scrawny. Plenty of them are fat too though.

Source: I'm a leftist nerd, carrying a few too many pounds, who doesn't eat meat.

Have you been keeping logs of all the vegans you meet to come to this conclusion? This sounds like anecdotal evidence at best.

Of course - unless one is profoundly autistic, I assume one remembers basic facts about people one interacts with, such as "do they look like shit" and "do they have any weird dietary things".