On Windows 7. QGIS 2.6.0 from OSGEO network installer 64bit (and checked with latest dev version)
I am preparing a print composer map displaying only vector data (shp). Data in the map canvas and the print composer are of same epsg 32624 projection. Input shp data is c. 1-2 mb. The map in the print composer is displaying a subset of the map data. In item properties of the map item, I have pressed the "View extent in map canvas" button to assure, that the map canvas and map item is of roughly the same extent. I've given the map item a thick frame to distinguish it clearly.
The SVG output is 76 kb of size, so far from huge. Nevertheless, the output SVG displays vector map data outside of the map item frame. The problem does not occur in image (png etc.) or pdf output, and neither using raster input data, eg. GeoTIFF.
I've searched the qgis tag, but haven't found a completely similar issue reported, as e.g. memory doesn't seem to be a problem as the input or output file sizes a both very small.