[MapProxy] Trouble reprojecting EPSG:4326 WMTS to EPSG:3857

[Luke Roth]

I'm trying to use WMTS data provided by NASA
(https://earthdata.nasa.gov/about-eosdis/system-description/global-imagery-browse-services-gibs)
that is available in EPSG:4326.  To overlay it with my base map, I'm 
following the example in the documentation to reproject it to 
EPSG:3857.  However, the resulting map is coming out scaled up by 125%.  
The only way I've discover to fix this is by scaling the bbox in the 
grid definition down by 125%, but that means that the last 25% of the 
globe is chopped off.  Has anyone else encountered this kind of 
problem?  Here's my conf.yaml file:

  services:
   demo:
   wms:
     md:
       title: GIBS MapProxy

grids:
   wm_grid: { base: GLOBAL_WEBMERCATOR }
   wgs84_grid: { base: GLOBAL_GEODETIC, tile_size: [512, 512], origin: 
nw, bbox: [-180, -54, 108, 90], bbox_srs: 'EPSG:4326' }

layers:
   - {name: terra_corr_refl_2013-11-22_output, title: 
MODIS_Terra_CorrectedReflectance_TrueColor 2013-11-22, sources: 
[terra_corr_refl_2013-11-22_cache]}

caches:
   terra_corr_refl_2013-11-22_cache: { disable_storage: false, grids: 
[wm_grid], sources: [terra_corr_refl_2013-11-22_input] }
   terra_corr_refl_2013-11-22_input: { disable_storage: true, grids: 
[wgs84_grid], sources: [terra_corr_refl_2013-11-22_source] }

sources:
   terra_corr_refl_2013-11-22_source:
     type: tile
     grid: wgs84_grid
     url: 
http://map1.vis.earthdata.nasa.gov/wmts-geo/MODIS_Terra_CorrectedReflectance_TrueColor/default/2013-11-22/EPSG4326_250m/%(z)s/%(y)s/%(x)s.jpeg

Thanks,
Luke Roth