I am from Kathmandu and lived in NYC for a decade. I believe it.

Much of KTM population comes from people from other parts of the country. KTM is not just an administrative hub being a capital, but it’s the biggest city with the biggest airports and other conveniences not afforded in other part of the county (though there are many cities that have sprung up in recent years). During Dashamin, Nepali’s biggest festival, these residents go back to their home town and population of Kathmandu dips down in orders of magnitude