Yeah, the formatting for affiliate buy buttons is "unlucky", and it gets even worse as soon as you activate the social sharing buttons in WooCommerce. I reported this earlier and a minor CSS fix was promised for one of the next updates.
In the meantime:
I suggest using Firebug to inspect the relevant CSS classes. Then you can play around with the CSS settings for this button in Firebug and watch the changes live - until it fits your needs.
I wanted mine to be bigger and have more whitespace around it so it sticks out, so I added some custom CSS and now it is ok.
It is not that difficult - and getting the hang of Firebug and basic CSS is unfortunately a necessity anyways, usually no pre-made theme does/shows everything exactly the way you want it to, so modifications have to be made....
cheers and good luck!