first, yes our lawn mowers do pollute a lot...... More than 40 cars...hard to say. also have to remember that most of the test and data are based on 21" style machines. What needs to be considered, at least from a landscape standpoint, does a 27hp kohler which only spend 15 minutes on yard running produce less pollutant than the 21"machine that spends and hour on the same parcel of land...My guess would be, Yes the larger engine is more efficient and most likely pollutes less. plus on the large engine equipment you will begin to have provisions for Catalytic converters and such to decrease the pollution even more.
I don't buy into global warming is casued soley by pollution, but I do believe that pollution is not doing us any favors and can be a contributing factor
. Pollution is not a good thing...And anything reasonable we can do to lessen it, IMO is a good idea...