Wide borders look weird for small polygons

I am experiencing this kind of visual defects for small polygons. These, in particular, are the small islands in the Pacific. As you see, instead of being uniform and nice, the borders have turned into some sort of hollowed out dried bubble marks.

The .mss file used to generate this style for mapnik with kosmtik is very simple:

 Map { background-color: #bbffff; }     
 #countries { polygon-fill: #ffffba; line-color: #8dd; line-join: round; line-cap: round; line-width: 14; }

I have played with the line-join and line-cap properties, but this is the best result I have got and it is not enough.

Can this problem be solved at all? Am I missing something? Must something be done differently?