How can I stop Jetpack mobile theme from using full size featured images?

It turns out Minileven (the mobile theme that comes bundled with Jetpack) uses the main theme’s default post_thumbnail_size, which is set using set_post_thumbnail_size. Adding this line to my main theme’s functions.php inside a function called through the after_theme_setup action helped solve the problem:

set_post_thumbnail_size( 600, 400, true );