Indeed, the 'Sofort' button should be the same size as other options.
Now it seems like this bunq.me service is aimed at dutch market only, while we do have so many other european countries and customers already.
As bunq has intentions to become a internationall/global bank, we should not make a difference in which payment methods are presented.
Maybe a puldownmenu would be a solution when there are more available?
https://bunq.me/anyway