If I understand the problem, what I would do, using photshop is:

- select the shape of, for example, the island you wish to enlarge;

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- select it and copy it to a new layer;

- use free transform (CTRL - T), say to 500% on both axis, this enlarge your isle;

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- select the shape, fill in white, select inverse fill in black

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- create a new layer, filter -> clouds -> render clouds (make sure foreground color is black and background is white hitting D);

- create new fill layer - threshold, adjust until you have something like this (on the cloud filtered layer adjust the opacity too see the shape of the island);

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- on the cloud filtered layer paint in black or white whith a soft, low opacity;

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- on the cloud filtered layer copy merged and then paste on another layer (or another file)


hope that answers your question...