In addition to rysloan suggestions you can use Mapshaper and QGIS to extract the state boundaries. Both of these are also free. To get just the us-states follows these steps:
- Upload the us.json file to Mapshaper;
- Export it as a shape file;
- Unpack the zip and you'll notice the Mapshaper has already split your topojson into 3 shape files one of which is us-states;
- Upload the us-states.shp and us-states.shx to Mapshaper (you'll have to refresh Mapshaper to get rid of the orginal us.json file first); and
- Save as topojson and you'll have a file 58kb in size.