I assume that you are using ThemeKit?
If so can you check a few things:
Here are ym config.yml content:
development: password: c12538e8d0442b68454b39e7******** theme_id: "7740820****" store: t1996.myshopify.com
I am using themekit latest version 1.0.1
I am not trying to upload a big file. I just get theme files using themid and then try to run the command theme watch so that it update automatically after a change in any file.