Light duty covers most of the 10-20' UPS/Fedex/whatever trucks for last mile urban delivery which is far less efficient than delivery to a warehouse. Without more data on fuel consumption by gross weight, it's impossible to separate industrial delivery from personal travel/commuting.
You're probably right because the max GVW on a single axle is 20,000 lbs but my source is the USDOT sticker on the license plates of several Fedex and UPS trucks that deliver to my suburban address and my parents' exurban address.
ORLY, source?
According to epa.gov [0] light-duty vehicles constitute 57% of transportation sector GHG emissions. Medium-and Heavy-duty trucks: 26%
[0] https://www.epa.gov/greenvehicles/fast-facts-transportation-...