Image replacement resizing issue

Hi all

InDesign has just started doing somethign incredibly annoying and I don't know if I have changed a setting, or if something has changed with the new update.

I work with a lot of complex icon style elements that I have created. They are set up to exactly the same size and specification so that I can swop in and out differing colour versions depending on the content. e.g. I have a client who has very different RGB and CMYK colour palettes, so I have the icons in both colour sets so that when I am converting one to the other (for print lets say) all I need to do is relink the image and the jobs done. BUT, recently when I relink the image the replacement comes in at a different size by default. Before it would just replace exactly - but now I have to manually select 'media' from the options box (this is the only one that works correctly). 

Does anyone know how to change the default settings so that it goes back to being automatic?

Many thanks