MRXS huge white borders

[Tom N.]

I am using "vips dzsave" pyramid builder to convert an MRXS slide to 
Zoomify on Ubuntu. Vips uses OpenSlide to read the MRXS slide.  The 
conversion is working fine, but the resulting Zoomify slide has huge 
white borders. For comparison, I also run a converter on Windows which 
is based on the native library supplied by the vendor. The resulting 
Zoomify slide from the Windows based converter doesn't have the white 
borders, and has therefore half the number of tiles.

Here is the ImageProperties.xml produced by the native Windows converter:

<IMAGE_PROPERTIES WIDTH="44544" HEIGHT="61952" NUMTILES="56242" 
NUMIMAGES="1" VERSION="1.8" TILESIZE="256" />

Here is the one produced by vips dzsave based on OpenSlide on Ubuntu:

<IMAGE_PROPERTIES WIDTH="62084" HEIGHT="141394" NUMTILES="179578" 
NUMIMAGES="1" VERSION="1.8" TILESIZE="256" />

Does anyone experience the same? Could this be a problem of OpenSlide or 
of VIPS pyramid builder?

Thank you for any input.
Tom